The lateral parts of the ventral nerve cords are now at their maximum of

separation (Pl. 32, fig. 18, _v.g._). Considerable differentiation has

already set in in the constitution of the ganglia themselves, which are

composed of an outer mass of ganglion cells enclosing a kernel of nerve

fibres, which lie on the inner side and connect the successive ganglia.

There are still distinct thoracic and abdominal ganglia for each segment,

and there is also a pair of separate ganglion for the cheliceræ, which

assists, however, in forming the oesophageal commissures.

The thickenings of the præoral lobe which form the supra-oesophageal

ganglia are nearly though not quite separated from the epiblast. The

semicircular grooves of the earlier stages are now deeper than before, and

are well shewn in sections nearly parallel to the outer anterior surface of

the ganglion (Pl. 32, fig. 19). The supra-oesophageal ganglia are still

entirely formed of undifferentiated cells, and are without commissural

tissue like that present in the ventral ganglia.

The stomodæum has considerably increased in length, and the proctodæum has

become formed as a short, posteriorly directed involution of the epiblast.

I have seen traces of what I believe to be two outgrowths from it, which

form the Malpighian bodies.

The next stage constitutes (Pl. 30, fig. 9) the last which requires to be

dealt with so far as the external features are concerned. The yolk has now

mainly passed into the abdomen, and the constriction separating the thorax

and abdomen has begun to appear. The yolk-sack has become absorbed, so that

the two halves of the ventral plate in the thorax are no longer widely

divaricated. The limbs have to a large extent acquired their permanent

structure, and the rings of which they are formed in the earlier stages are

now replaced by definite joints. A delicate cuticle has become formed,

which is not figured in my sections. The four rudimentary appendages have

disappeared, unless, which seems to me in the highest degree improbable,

they remain as the spinning mammillæ, two pairs of which are now present.

Behind is the anal lobe, which is much smaller and less conspicuous than in

the previous stage. The spinnerets and anal lobe are shewn as five papillæ

in Pl. 30, fig. 9. Dorsally the heart is now very conspicuous, and in front

of the cheliceræ may be seen the supra-oesophageal ganglia.

The indifferent mesoblast has now to a great extent become converted into

the permanent tissues. On the dorsal surface there was present in the last

stage a great mass of unformed mesoblast cells. This mass of cells has now

become divided into a somatic and splanchnic layer (Pl. 32, fig. 22). It

has, moreover, in the abdominal region at any rate, become divided up into

somites. At the junction between the successive somites the splanchnic

mesoblast on each side of the abdomen dips down into the yolk and forms a

septum (Pl. 32, fig. 22, _s_). The septa so formed, which were first

described by Barrois, are not complete. The septa of the two sides do not,

in the first place, quite meet along the median dorsal or ventral lines,

and in the second place they only penetrate the yolk for a certain

distance. Internally they usually end in a thickened border.

Along the line of insertion of each of these septa there is developed a

considerable space between the somatic and splanchnic layers of mesoblast.

The parts of the body-cavity so established are transversely directed

channels passing from the heart outwards. They probably constitute the

venous spaces, and perhaps also contain the transverse aortic branches.

In the intervals between these venous spaces the somatic and splanchnic

layers of mesoblast are in contact with each other.

I have not been able to work out satisfactorily the later stages of

development of the septa, but I have found that they play an important part

in the subsequent development of the abdomen. In the first place they send

off lateral offshoots, which unite the various septa together, and divide

up the cavity of the abdomen into a number of partially separated

compartments. There appears, however, to be left a free axial space for the

alimentary tract, the mesoblastic walls of which are, I believe, formed

from the septa.

At the present stage the splanchnic mesoblast, apart from the septa, is a

delicate membrane of flattened cells (fig. 22, _sp_). The somatic mesoblast

is thicker, and is formed of scattered cells (_so_).

The somatic layer is in part converted, in the posterior region of the

abdomen, into a delicate layer of longitudinal muscles, the fibres of which

are not continuous for the whole length of the body, but are interrupted at

the lines of junction of the successive segments. They are not present in

the anterior part of the abdomen. The longitudinal direction of these

fibres, and their division with myotomes, is interesting, since both these

characters, which are preserved in Scorpions, are lost in the abdomen of

the adult Spider.

The original mesoblastic somites have undergone quite as important changes

as the dorsal mesoblast. In the abdominal region the somatic layer

constitutes two powerful bands of longitudinal muscles, inserted anteriorly

at the root of the fourth ambulatory appendage, and posteriorly at the

spinning mammillæ. Between these two bands are placed the nervous bands.

The relation of these parts are shewn in the section in Pl. 32, fig. 20_d_,

which cuts the abdomen horizontally and longitudinally. The mesoblastic

bands are seen at _m._, and the nervous bands within them at _ab.g_. In the

thoracic region the part of the somatic layer in each limb is converted

into muscles, which are continued into dorsal and ventral muscles in the

thorax (vide fig. 20_c_). There are, in addition to these, intrinsic

transverse fibres on the ventral side of the thorax. Besides these muscles

there are in the thorax, attached to the suctorial extremity of the

stomodæum, three powerful muscles, which I believe to be derived from the

somatic mesoblast. One of these passes vertically down from the dorsal

surface, in the septum the commencement of which was described in the last

stage. The two other muscles are lateral, one on each side (Pl. 31,

fig. 20_c_.).

The heart has now, in most respects, reached its full development. It is

formed of an outer muscular layer, within which is a doubly-contoured

lining, containing nuclei at intervals, which is probably of the nature of

an epithelioid lining (Pl. 32, fig. 22, _ht_). In its lumen are numerous

blood-corpuscles (not represented in my figure). The heart lies in a space

bound below by the splanchnic mesoblast, and to the sides by the somatic

mesoblast. This space forms a kind of pericardium (fig. 22, _pc_), but

dorsally the heart is in contact with the epiblast. The arterial trunks

connected with it are fully established.

The nervous system has undergone very important changes.

In the abdominal region the ganglia of each side have fused together into a

continuous cord (fig. 21, _ab.g_). In fig. 20, in which the abdomen is cut

horizontally and longitudinally, there are seen the two abdominal cords

(_ab.g_) united by two transverse commissures; and I believe that there are

at this stage three or four transverse commissures at any rate, which

remain as indications of the separate ganglia, from the coalescence of

which the abdominal cords are formed. The two abdominal cords are parallel

and in close contact.

In the thoracic region changes of not less importance have taken place. The

ganglia are still distinct. The two cords formed of these ganglia are no

longer widely separated in median line, but meet, in the usual way, in the

ventral line. Transverse commissures have become established (fig. 20_c_)

between the ganglia of the two sides. There is as little trace at this, as

at the previous stages, of an ingrowth of epiblast, to form a median

portion of the central nervous system. Such a median structure has been

described by Hatschek for Lepidoptera, and he states that it gives rise to

the transverse commissures between the ganglia. My observations shew that

for the spider, at any rate, nothing of the kind is present.

As shewn in the longitudinal section (Pl. 32, fig. 21), the ganglion of the

cheliceræ has now united with the supra-oesophageal ganglion. It forms, as

is shewn in fig. 20_b_ (_ch.g._), a part of the oesophageal commissure, and

there is no sub-oesophageal commissure uniting the ganglia of the

cheliceræ, but the oesophageal ring is completed below by the ganglia of

the pedipalpi (fig. 20_c_, _pd.g._).

The supra-oesophageal ganglia have become completely separated from the

epiblast.

I have unfortunately not studied their constitution in the adult, so that I

cannot satisfactorily identify the parts which can be made out at this

stage.

I distinguish, however, the following regions:

(1) A central region containing the commissural part, and continuous below

with the ganglia of the cheliceræ.

(2) A dorsal region formed of two hemispherical lobes.

(3) A ventral anterior region.

The central region contains in its interior the commissural portion,

forming a punctiform, rounded mass in each ganglion. A transverse

commissure connects the two (vide fig. 20_b_).

The dorsal hemispherical lobes are derived from the part which, at the

earlier stage, contained the semicircular grooves. When the

supra-oesophageal ganglia become separated from the epidermis the cells

lining these grooves become constricted off with them, and form part of

these ganglia. Two cavities are thus formed in this part of the

supra-oesophageal ganglia. These cavities become, for the most part,

obliterated, but persist at the outer side of the hemispherical lobes

(figs. 20_a_ and 21).

The ventral lobe of the brain is a large mass shewn in longitudinal section

in fig. 21. It lies immediately in front of and almost in contact with the

ganglia of the cheliceræ.

The two hemispherical lobes agree in position with the fungiform body

(_pilzhutförmige Körpern_), which has attracted so much the attention of

anatomists, in the supra-oesophageal ganglia of Insects and Crustacea; but

till the adult brain of Spiders has been more fully studied it is not

possible to state whether the hemispherical lobes become fungiform bodies.

Hatschek[473] has described a special epiblastic invagination in the

supra-oesophageal ganglion of Bombyx, which is probably identical with the

semicircular groove of Spiders and Scorpions, but in the figure he gives

the groove does not resemble that in the Arachnida. A similar groove is

found in Peripatus, and there forms, as I have found, a large part of the

supra-oesophageal ganglia. It is figured by Moseley, _Phil. Trans._,

Vol. CLXIV. pl. lxxv, fig. 9.

Footnote 473: "Beiträge z. Entwick. d. Lepidopteren,"

_Jenaische Zeit._, Vol. XI. p. 124.

The stomodæum is considerably larger than in the last stage, and is lined

by a cuticle; it is a blind tube, the blind end of which is the suctorial

pouch of the adult. To this pouch are attached the vertical dorsal, and two

lateral muscles spoken of above.

The proctodæum (_pr._) has also grown in length, and the two Malpighian

vessels which grow out from its blind extremity (fig. 20_e_, _mp.g._) have

become quite distinct. The part now formed is the rectum of the adult. The

proctodæum is surrounded by a great mass of splanchnic mesoblast. The

mesenteron has as yet hardly commenced to be developed. There is, however,

a short tube close to the proctodæum (fig. 20_e_, _mes_), which would seem

to be the commencement of it. It ends blindly on the side adjoining the

rectum, but is open anteriorly towards the yolk, and there can be very

little doubt that it owes its origin to cells derived from the yolk. On its

outer surface is a layer of mesoblast.

From the condition of the mesenteron at this stage there can be but little

doubt that it will be formed, not on the surface, _but in the interior of

the yolk_. I failed to find any trace of an anterior part of the mesenteron

adjoining the stomodæum. In the posterior part of the thorax (vide

fig. 20_d_), there is undoubtedly no trace of the alimentary tract.

The presence of this rudiment shews that Barrois is mistaken in supposing

that the alimentary canal is formed entirely from the stomodæum and

proctodæum, which are stated by him to grow towards each other, and to meet

at the junction of the thorax and abdomen. My own impression is that the

stomodæum and proctodæum have reached their full extension at the present

stage, and that both the stomach in the thorax and the intestine in the

abdomen are products of the mesenteron.

The yolk retains its earlier constitution, being divided into polygonal

segments, formed of large yolk vesicles. The nuclei are more numerous than

before. In the thorax the yolk is anteriorly divided into two lobes by the

vertical septum, which contains the vertical muscle of the suctorial pouch.

In the posterior part of the thorax it is undivided.

I have not yet been able clearly to make out the eventual fate of the yolk.

At a subsequent stage, when the cavity of the abdomen is cut up into a

series of compartments by the growth of the septa, described above, the

yolk fills these compartments, and there is undoubtedly a proliferation of

yolk cells round the walls of these compartments. It would not be

unreasonable to conclude from this that the compartments were destined to

form the hepatic cæca, each cæcum being enclosed in a layer of splanchnic

mesoblast, and its hypoblastic wall being derived from the yolk cells. I

think that this hypothesis is probably correct, but I have met with some

facts which made me think it possible that the thickenings at the ends of

the septa, visible in Pl. 32, fig. 22, were the commencing hepatic cæca.

I must, in fact, admit that I have hitherto failed to work out

satisfactorily the history of the mesenteron and its appendages. The firm

cuticle of young spiders is an obstacle both in the way of making sections

and of staining, which I have not yet overcome.

_General Conclusions._

Without attempting to compare at length the development of the spiders with

that of other Arthropoda, I propose to point out a few features in the

development of spiders, which appear to shew that the Arachnida are

undoubtedly more closely related to the other Tracheata than to the

Crustacea.

The whole history of the formation of the mesoblast is very similar to that

in insects. The mesoblast in both groups is formed by a thickening of the

median line of the ventral plate (germinal streak).

In insects there is usually formed a median groove, the walls of which

become converted into a plate of mesoblast. In spiders there is no such

groove, but a median keel-like thickening of the ventral plate (Pl. 31,

fig. 11), is very probably an homologous structure. The unpaired plate of

mesoblast formed in both insects and Arachnida is exactly similar, and

becomes divided, in both groups, into two bands, one on each side of the

middle line. Such differences as there are between Insects and Arachnida

sink into insignificance compared with the immense differences in the

origin of the mesoblast between either group, and that in the Isopoda, or,

still more, the Malacostraca and most Crustacea. In most Crustacea we find

that the mesoblast is budded off from the walls of an invagination, which

gives rise to the mesenteron.

In both spiders and Myriopoda, and probably insects, the mesoblast is

subsequently divided into somites, the lumen of which is continued into the

limbs. In Crustacea mesoblastic somites have not usually been found, though

they appear occasionally to occur, _e.g._ Mysis, but they are in no case

similar to those in the Tracheata.

In the formation of the alimentary tract, again, the differences between

the Crustacea and Tracheata are equally marked, and the Arachnida agree

with the Tracheata. There is generally in Crustacea an invagination, which

gives rise to the mesenteron. In Tracheata this never occurs. The

proctodæum is usually formed in Crustacea before or, at any rate, not later

than the stomodæum[474]. The reverse is true for the Tracheata. In

Crustacea the proctodæum and stomodæum, especially the former, are very

long, and usually give rise to the greater part of the alimentary tract,

while the mesenteron is usually short.

Footnote 474: If Grobben's account of the development of Moina

is correct this statement must be considered not to be

universally true.

In the Tracheata the mesenteron is always considerable, and the proctodæum

is always short. The derivation of the Malpighian bodies from the

proctodæum is common to most Tracheata. Such organs are not found in the

Crustacea.

With reference to other points in my investigations, the evidence which I

have got that the cheliceræ are true postoral appendages supplied in the

embryo from a distinct postoral ganglion, confirms the conclusions of most

previous investigators, and shews that these appendages are equivalent to

the mandibles, or possibly the first pair of maxillæ of other Tracheata.

The invagination, which I have found, of part of a groove of epiblast in

the formation of the supra-oesophageal ganglia is of interest, owing to the

wide extension of a similar occurrence amongst the Tracheata.

The wide divarication of the ventral nerve cords in the embryo renders it

easy to prove that there is no median invagination of epiblast between

them, and supports Kleinenberg's observations on Lumbricus as to the

absence of this invagination. I have further satisfied myself as to the

absence of such an invagination in Peripatus. It is probable that Hatschek

and other observers who have followed him are mistaken in affirming the

existence of such an invagination in either the Chætopoda or the

Arthropoda.

The observations recorded in this paper on the yolk cells and their

derivations are, on the whole, in close harmony with the observations of

Dohrn, Bobretzky, and Graber, on Insects. They shew, however, that the

first formed mesoblastic plate does not give rise to the whole of the

mesoblast, but that during the whole of embryonic life the mesoblast

continues to receive accessions of cells derived from the cells of the

yolk.

_Araneina._

1. Balbiani, "Mémoire sur le Développement des Araneides," _Ann. Sci.

Nat._, series v, Vol. XVII. 1873.

2. J. Barrois, "Recherches s. l. Développement des Araignées," _Journal de

l'Anat. et de la Physiol._, 1878.

3. E. Claparède, _Recherches s. l'Evolution des Araignées_, Utrecht, 1860.

4. Herold, _De Generatione Araniorum in Ovo_, Marburg, 1824.

5. H. Ludwig, "Ueb. d. Bildung des Blastoderm bei d. Spinnen," _Zeit. f.

wiss. Zool._, Vol. XXVI. 1876.

EXPLANATION OF PLATES 30, 31, AND 32.

PLATE 30.

COMPLETE LIST OF REFERENCE LETTERS.

_ch._ Cheliceræ. _ch.g._ Ganglion of cheliceræ. _c.l._ Caudal lobe. _p.c._

Primitive cumulus. _pd._ Pedipalpi. _pr.l._ Præoral lobe. _pp_{1}. _pp_{2}.

_etc._ Provisional appendages. _sp._ Spinnerets. _st._ Stomodæum.

I-IV. Ambulatory appendages. 1-16. Postoral segments.

Fig. 1. Ovum, with primitive cumulus and streak proceeding from it.

Fig. 2. Somewhat later stage, in which the primitive cumulus is still

visible. Near the opposite end of the blastoderm is a white area, which is

probably the rudiment of the procephalic lobe.

Fig. 3_a_ and 3_b_. View of an embryo from the ventral surface and from the

side when six segments have become established.

Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of

the appendages become visible.

Fig. 5. Embryo ideally unrolled at the stage when all the appendages have

become established.

Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed

from the side.

Fig. 7. Later stage, viewed from the side.

Fig. 7_a_. Same embryo as fig. 7, ideally unrolled.

Figs. 8_a_ and 8_b_. View from the ventral surface and from the side of an

embryo, after the ventral flexure has considerably advanced.

Fig. 9. Somewhat older embryo, viewed from the ventral surface.

PLATES 31 AND 32.

COMPLETE LIST OF REFERENCE LETTERS.

_ao._ Aorta. _ab.g._ Abdominal nerve cord. _ch._ Cheliceræ. _ch.g._

Ganglion of cheliceræ. _ep._ Epiblast. _hs._ Hemispherical lobe of

supra-oesophageal ganglion. _ht._ Heart. _l.l._ Lower lip. _m._ Muscles.

_me._ Mesoblast. _mes._ Mesenteron. _mp.g._ Malpighian tube. _ms._

Mesoblastic somite. _oe._ OEsophagus. _p.c._ Pericardium. _pd._ Pedipalpi.

_pd.g._ Ganglion of pedipalpi. _pr._ Proctodæum (rectum). _pr.c._ Primitive

cumulus. _s._ Septum in abdomen. _so._ Somatopleure. _sp._ Splanchnopleure.

_st._ Stomodæum. _su._ Suctorial apparatus. _su.g._ Supra-oesophageal

ganglion. _th. g._ Thoracic ganglion. _v.g._ Ventral nerve cord. _y.c._

Cells derived from yolk. _yk._ Yolk. _y.n._ Nuclei of yolk cells.

I_g_-IV_g_. Ganglia of ambulatory limbs. 1-16. Postoral segments.

Fig. 10. Section through an ovum, slightly younger than fig. 1. Shewing the

primitive cumulus and the columnar character of the cells of one half of

the blastoderm.

Fig. 11. Section through an embryo of the same age as fig. 2. Shewing the

median thickening of the blastoderm.

Fig. 12. Transverse section through the ventral plate of a somewhat older

embryo. Shewing the division of the ventral plate into epiblast and

mesoblast.

Fig. 13. Section through the ventral plate of an embryo of the same age as

fig. 3, shewing the division of the mesoblast of the ventral plate into two

mesoblastic bands.

Fig. 14. Transverse section through an embryo of the same age as fig. 5,

passing through an abdominal segment above and a thoracic segment below.

Fig. 15. Longitudinal section slightly to one side of the middle line

through an embryo of the same age.

Fig. 16. Transverse section through the ventral plate in the thoracic

region of an embryo of the same age as fig. 7.

Fig. 17. Transverse section through the procephalic lobes of an embryo of

the same age. _gr._ Section of hemicircular groove in procephalic lobe.

Fig. 18. Transverse section through the thoracic region of an embryo of the

same age as fig. 8.

Fig. 19. Section through the procephalic lobes of an embryo of the same

age.

Fig. 20_a_, _b_, _c_, _d_, _e_. Five sections through an embryo of the same

age as fig. 9. _a_ and _b_ are sections through the procephalic lobes, _c_

through the front part of the thorax. _d_ cuts transversely the posterior

parts of the thorax, and longitudinally and horizontally the ventral

surface of the abdomen. _e_ cuts the posterior part of the abdomen

longitudinally and horizontally, and shews the commencement of the

mesenteron.

Fig. 21. Longitudinal and vertical section of an embryo of the same age.

The section passes somewhat to one side of the middle line, and shews the

structure of the nervous system.

Fig. 22. Transverse section through the dorsal part of the abdomen of an

embryo of the same stage as fig. 9.

XVIII. ON THE SPINAL NERVES OF AMPHIOXUS[475].

Footnote 475: From the _Quarterly Journal of Microscopical

Science_, Vol. XX. 1880.

In an interesting memoir devoted to the elucidation of a series of points

in the anatomy and development of the Vertebrata, Schneider[476] has

described what he believes to be motor nerves in Amphioxus, which spring

from the anterior side of the spinal cord. According to Schneider these

nerves have been overlooked by all previous observers except Stieda.

Footnote 476: _Beiträge z. Anat. u. Entwick. d. Wirbelthiere_,

Berlin, 1879.

I[477] myself attempted to shew some time ago that anterior roots were

absent in Amphioxus; and in some speculations on the cranial nerves, I

employed this peculiarity of the nervous system of Amphioxus to support a

view that Vertebrata were primitively provided only with nerves of mixed

function springing from the posterior side of the spinal cord. Under these

circumstances, Schneider's statement naturally attracted my attention, and

I have made some efforts to satisfy myself as to its accuracy. The nerves,

as he describes them, are very peculiar. They arise from a number of

distinct roots in the hinder third of each segment. They form a flat

bundle, of which part passes upwards and part downwards. When they meet the

muscles they bend backwards, and fuse with the free borders of the

muscle-plates. The fibres, which at first sight appear to form the nerve,

are, however, transversely striated, and are regarded by Schneider as

muscles; and he holds that each muscle-plate sends a process to the edge of

the spinal cord, which there receives its innervation. A considerable body

of evidence is requisite to justify a belief in the existence of such very

extraordinary and unparalleled motor nerves; and for my part I cannot say

that Schneider's observations are convincing to me. I have attempted to

repeat his observations, employing the methods he describes.

Footnote 477: "On the Spinal Nerves of Amphioxus," _Journ. of

Anat. and Phys._ Vol. X. 1876. [This edition, No. IX. p. 197.]

In the first place, he states that by isolating the spinal cord by boiling

in acetic acid, the anterior roots may be brought into view as numerous

conical processes of the spinal cord in each segment. I find by treating

the spinal cord in this way, that processes more or less similar, but more

irregular than those which he figures, are occasionally present; but I

cannot persuade myself that they are anything but parts of the sheath of

the spinal cord which is not completely dissolved by treatment with acetic

acid. By treatment with nitric acid _no such processes are to be seen_,

though the whole length and very finest branches of the posterior nerves

are preserved.

By treating with nitric acid and clarifying by oil of cloves, and

subsequently removing one half of the body so as to expose the spinal cord

_in sitû_, the origin and distribution of the posterior nerves is very

clearly exhibited. But I have failed to detect any trace of the anterior

nerve-roots. Horizontal section, which ought also to bring them clearly

into view, failed to shew me anything which I could interpret as such. I

agree with Schneider that a process of each muscle-plate is prolonged up to

the anterior border of the spinal cord, but I can find no trace of a

connection between it and the cord.

Schneider has represented a transverse section in which the anterior nerves

are figured. I am very familiar with an appearance in section such as that

represented in his figure, but I satisfied myself when I previously studied

the nerves in Amphioxus, that the body supposed to be a nerve by Schneider

was nothing else than part of the intermuscular septum, and after

re-examining my sections I see no reason to alter my view.

A very satisfactory proof that the ventral nerves do not exist would be

found, if it could be established that the dorsal nerves contained both

motor and sensory fibres. So far I have not succeeded in proving this; I

have not, however, had fresh specimens to assist me in the investigation.

Langerhans[478], whose careful observations appear to me to have been

undervalued by Schneider, figures a branch distributed to the muscles,

which passes off from the dorsal roots. Till the inaccuracy of this

observation is demonstrated, the balance of evidence appears to me to be

opposed to Schneider's view.

Footnote 478: _Archiv f. Mikros. Anatomie_, Vol. XII.

XIX. ADDRESS TO THE DEPARTMENT OF ANATOMY AND PHYSIOLOGY OF THE BRITISH

ASSOCIATION, 1880.

In the spring of the present year, Professor Huxley delivered an address at

the Royal Institution, to which he gave the felicitous title of '_The

coming of age of the origin of species_.' It is, as he pointed out,

twenty-one years since Mr Darwin's great work was published, and the

present occasion is an appropriate one to review the effect which it has

had on the progress of biological knowledge.

There is, I may venture to say, no department of biology the growth of

which has not been profoundly influenced by the Darwinian theory. When

Messrs Darwin and Wallace first enunciated their views to the scientific

world, the facts they brought forward seemed to many naturalists

insufficient to substantiate their far-reaching conclusions. Since that

time an overwhelming mass of evidence has, however, been rapidly

accumulating in their favour. Facts which at first appeared to be opposed

to their theories have one by one been shewn to afford striking proofs of

their truth. There are at the present time but few naturalists who do not

accept in the main the Darwinian theory, and even some of those who reject

many of Darwin's explanations still accept the fundamental position that

all animals are descended from a common stock.

To attempt in the brief time which I have at my disposal to trace the

influence of the Darwinian theory on all the branches of anatomy and

physiology would be wholly impossible, and I shall confine myself to an

attempt to do so for a small section only. There is perhaps no department

of Biology which has been so revolutionised, if I may use the term, by the

theory of animal evolution, as that of Development or Embryology. The

reason of this is not far to seek. According to the Darwinian theory, the

present order of the organic world has been caused by the action of two

laws, known as the laws of heredity and of variation. The law of heredity

is familiarly exemplified by the well-known fact that offspring resemble

their parents. Not only, however, do the offspring belong to the same

species as their parents, but they inherit the individual peculiarities of

their parents. It is on this that the breeders of cattle depend, and it is

a fact of every-day experience amongst ourselves. A further point with

reference to heredity to which I must call your attention is the fact that

the characters, which display themselves at some special period in the life

of the parent, are acquired by the offspring at a corresponding period.

Thus, in many birds the males have a special plumage in the adult state.

The male offspring is not, however, born with the adult plumage, but only

acquires it when it becomes adult.

The law of variation is in a certain sense opposed to the law of heredity.

It asserts that the resemblance which offspring bear to their parents is

never exact. The contradiction between the two laws is only apparent. All

variations and modifications in an organism are directly or indirectly due

to its environments; that is to say, they are either produced by some

direct influence acting upon the organism itself, or by some more subtle

and mysterious action on its parents; and the law of heredity really

asserts that the offspring and parent would resemble each other if their

environments were the same. Since, however, this is never the case, the

offspring always differ to some extent from the parents. Now, according to

the law of heredity, every acquired variation tends to be inherited, so

that, by a summation of small changes, the animals may come to differ from

their parent stock to an indefinite extent.

We are now in a position to follow out the consequences of these two laws

in their bearing on development. Their application will best be made

apparent by taking a concrete example. Let us suppose a spot on the surface

of some very simple organism to become, at a certain period of life,

pigmented, and therefore to be especially sensitive to light. In the

offspring of this form, the pigment-spot will reappear at a corresponding

period; and there will therefore be a period in the life of the offspring

during which there is no pigment-spot, and a second period in which there

is one. If a naturalist were to study the life-history, or, in other words,

the embryology of this form, this fact about the pigment-spot would come to

his notice, and he would be justified, from the laws of heredity, in

concluding that the species was descended from an ancestor without a

pigment-spot, because a pigment-spot was absent in the young. Now, we may

suppose the transparent layer of skin above the pigment-spot to become

thickened, so as gradually to form a kind of lens, which would throw an

image of external objects on the pigment-spot. In this way a rudimentary

eye might be evolved out of the pigment-spot. A naturalist studying the

embryology of the form with this eye would find that the pigment-spot was

formed before the lens, and he would be justified in concluding, by the

same process of reasoning as before, that the ancestors of the form he was

studying first acquired a pigment-spot and then a lens. We may picture to

ourselves a series of steps by which the simple eye, the origin of which I

have traced, might become more complicated; and it is easy to see how an

embryologist studying the actual development of this complicated eye would

be able to unravel the process of its evolution.

The general nature of the methods of reasoning employed by embryologists,

who accept the Darwinian theory, is exemplified by the instance just given.

If this method is a legitimate one, and there is no reason to doubt it, we

ought to find that animals, in the course of their development, pass

through a series of stages, in each of which they resemble one of their

remote ancestors; but it is to be remembered that, in accordance with the

law of variation, there is a continual tendency to change, and that the

longer this tendency acts the greater will be the total effect. Owing to

this tendency, we should not expect to find a perfect resemblance between

an animal, at different stages of its growth, and its ancestors; and the

remoter the ancestors, the less close ought the resemblance to be. In

spite, however, of this limitation, it may be laid down as one of the

consequences of the law of inheritance that every animal ought, in the

course of its individual development, to repeat with more or less fidelity

the history of its ancestral evolution.

A direct verification of this proposition is scarcely possible. There is

ample ground for concluding that the forms from which existing animals are

descended have in most instances perished; and although there is no reason

why they should not have been preserved in a fossil state, yet, owing to

the imperfection of the geological record, palæontology is not so often of

service as might have been hoped.

While, for the reasons just stated, it is not generally possible to prove

by direct observation that existing forms in their embryonic state repeat

the characters of their ancestors, there is another method by which the

truth of this proposition can be approximately verified.

A comparison of recent and fossil forms shews that there are actually

living at the present day representatives of a considerable proportion of

the groups which have in previous times existed on the globe, and there are

therefore forms allied to the ancestors of those living at the present day,

though not actually the same species. If therefore it can be shewn that the

embryos of existing forms pass through stages in which they have the

characters of more primitive groups, a sufficient proof of our proposition

will have been given.

That such is often the case is a well-known fact, and was even known before

the publication of Darwin's works. Von Baer, the greatest embryologist of

the century, who died at an advanced age but a few years ago, discussed the

proposition at considerable length in a work published between the years

1830 and 1840. He came to the conclusion that the embryos of higher forms

never actually resemble lower forms, but only the embryos of lower forms;

and he further maintained that such resemblances did not hold at all, or

only to a very small extent, beyond the limits of the larger groups. Thus

he believed that, though the embryos of Vertebrates might agree amongst

themselves, there was no resemblance between them and the embryos of any

invertebrate group. We now know that these limitations of Von Baer do not

hold good, but it is to be remembered that the meaning _now_ attached by

embryologists to such resemblances was quite unknown to him.

These preliminary remarks will, I trust, be sufficient to demonstrate how

completely modern embryological reasoning is dependent on the two laws of

inheritance and variation, which constitute the keystones of the Darwinian

theory.

Before the appearance of the _Origin of Species_ many very valuable

embryological investigations were made, but the facts discovered were to

their authors merely so many ultimate facts, which admitted of being

classified, but could not be explained. No explanation could be offered of

why it is that animals, instead of developing in a simple and

straightforward way, undergo in the course of their growth a series of

complicated changes, during which they often acquire organs which have no

function, and which, after remaining visible for a short time, disappear

without leaving a trace.

No explanation, for instance, could be offered of why it is that a frog in

the course of its growth has a stage in which it breathes like a fish, and

then why it is like a newt with a long tail, which gradually becomes

absorbed, and finally disappears. To the Darwinian the explanation of such

facts is obvious. The stage when the tadpole breathes by gills is a

repetition of the stage when the ancestors of the frog had not advanced in

the scale of development beyond a fish, while the newt-like stage implies

that the ancestors of the frog were at one time organized very much like

the newts of to-day. The explanation of such facts has opened out to the

embryologist quite a new series of problems. These problems may be divided

into two main groups, technically known as those of phylogeny and those of

organogeny. The problems of phylogeny deal with the genealogy of the animal

kingdom. A complete genealogy would form what is known as a natural

classification. To attempt to form such a classification has long been the

aim of a large number of naturalists, and it has frequently been attempted

without the aid of embryology. The statements made in the earlier part of

my address clearly shew how great an assistance embryology is capable of

giving in phylogeny; and as a matter of fact embryology has been during the

last few years very widely employed in all phylogenetic questions, and the

results which have been arrived at have in many cases been very striking.

To deal with these results in detail would lead me into too technical a

department of my subject; but I may point out that amongst the more

striking of the results obtained _entirely_ by embryological methods is the

demonstration that the Vertebrata are not, as was nearly universally

believed by older naturalists, separated by a wide gulf from the

Invertebrata, but that there is a group of animals, known as the Ascidians,

formerly united with the Invertebrata, which are now universally placed

with the Vertebrata.

The discoveries recently made in organogeny, or the genesis of organs, have

been quite as striking, and in many respects even more interesting, than

those in phylogeny, and I propose devoting the remainder of my address to a

history of results which have been arrived at with reference to the origin

of the nervous system.

To render clear the nature of these results I must say a few words as to

the structure of the animal body. The body is always built of certain

pieces of protoplasm, which are technically known to biologists as cells.

The simplest organisms are composed either of a single piece of this kind,

or of several similar pieces loosely aggregated together. Each of these

pieces or cells is capable of digesting and assimilating food, and of

respiring; it can execute movements, and is sensitive to external stimuli,

and can reproduce itself. All the functions of higher animals can, in fact,

be carried on in this single cell. Such lowly organized forms are known to

naturalists as the Protozoa. All other animals are also composed of cells,

but these cells are no longer complete organisms in themselves. They

exhibit a division of labour: some carrying on the work of digestion; some,

which we call nerve-cells, receiving and conducting stimuli; some, which we

call muscle-cells, altering their form--in fact, contracting in one

direction--under the action of the stimuli brought to them by the

nerve-cells. In most cases a number of cells with the same function are

united together, and thus constitute a tissue. Thus the cells which carry

on the work of digestion form a lining membrane to a tube or sack, and

constitute a tissue known as a secretory epithelium. The whole of the

animals with bodies composed of definite tissues of this kind are known as

the Metazoa.

A considerable number of early developmental processes are common to the

whole of the Metazoa.

In the first place every Metazoon commences its existence as a simple cell,

in the sense above defined; this cell is known as the ovum. The first

developmental process which takes place consists in the division or

segmentation of the single cell into a number of smaller cells. The cells

then arrange themselves into two groups or layers known to embryologists as

the _primary germinal layers_. These two layers are usually placed one

within the other round a central cavity. The inner of the two is called the

hypoblast, the outer the epiblast. The existence of these two layers in the

embryos of vertebrated animals was made out early in the present century by

Pander, and his observations were greatly extended by Von Baer and Remak.

But it was supposed that these layers were confined to vertebrated animals.

In the year 1849, and at greater length in 1859, Huxley demonstrated that

the bodies of all the polype tribe or Coelenterata--that is to say of the

group to which the common polype, jelly-fish and the sea-anemone

belong--were composed of two layers of cells, and stated that in his

opinion these two layers were homologous with the epiblast and hypoblast of

vertebrate embryos. This very brilliant discovery came before its time. It

fell upon barren ground, and for a long time bore no fruit. In the year

1866 a young Russian naturalist named Kowalevsky began to study by special

histological methods the development of a number of invertebrated forms of

animals, and discovered that at an early stage of development the bodies of

all these animals were divided into germinal layers like those in

vertebrates. Biologists were not long in recognizing the importance of

these discoveries, and they formed the basis of two remarkable essays, one

by our own countryman, Professor Lankester, and the other by a

distinguished German naturalist, Professor Haeckel, of Jena.

In these essays the attempt was made to shew that the stage in development

already spoken of, in which the cells are arranged in the form of two

layers enclosing a central cavity has an ancestral meaning, and that it is

to be interpreted to signify that all the Metazoa are descended from an

ancestor which had a more or less oval form, with a central digestive

cavity provided with a single opening, serving both for the introduction of

food and for the ejection of indigestible substances. The body of this

ancestor was supposed to have been a double-walled sack formed of an inner

layer, the hypoblast, lining the digestive cavity, and an outer layer, the

epiblast. To this form Haeckel gave the name of gastræa or gastrula.

There is every reason to think that Lankester and Haeckel were quite

justified in concluding that a form more or less like that just described

was the ancestor of the Metazoa; but the further speculations contained in

their essays as to the origin of this form from the Protozoa can only be

regarded as suggestive feelers, which, however, have been of great

importance in stimulating and directing embryological research. It is,

moreover, very doubtful whether there are to be found in the developmental

histories of most animals any traces of this gastræa ancestor, other than

the fact of their passing through a stage in which the cells are divided

into two germinal layers.

The key to the nature of the two germinal layers is to be found in Huxley's

comparison between them, and the two layers in the fresh-water polype and

the sea-anemone. The epiblast is the primitive skin, and the hypoblast is

the primitive epithelial wall of the alimentary tract.

In the whole of the polype group, or Coelenterata, the body remains through

life composed of the two layers, which Huxley recognized as homologous with

the epiblast and hypoblast of the Vertebrata; but in all the higher Metazoa

a third germinal layer, known as the mesoblast, early makes its appearance

between the two primary layers. The mesoblast originates as a

differentiation of one or of both the primary germinal layers; but although

the different views which have been held as to its mode of origin form an

important section of the history of recent embryological investigations, I

must for the moment confine myself to saying that from this layer there

take their origin--the whole of the muscular system, of the vascular

system, and of that connective-tissue system which forms the internal

skeleton, tendons, and other parts.

We have seen that the epiblast represents the skin or epidermis of the

simple sack-like ancestor common to all the Metazoa. In all the higher

Metazoa it gives rise, as might be expected, to the epidermis, but it gives

rise at the same time to a number of other organs; and, in accordance with

the principles laid down in the earlier part of my address, it is to be

concluded that _the organs so derived have been formed as differentiations

of the primitive epidermis_. One of the most interesting of recent

embryological discoveries is the fact that the nervous system is, in all

but a very few doubtful cases, derived from the epiblast. This fact was

made out for vertebrate animals by the great embryologist Von Baer; and the

Russian naturalist Kowalevsky, to whose researches I have already alluded,

shewed that this was true for a large number of invertebrate animals. The

derivation of the nervous system from the epiblast has since been made out

for a sufficient number of forms satisfactorily to establish the

generalization that it is all but universally derived from the epiblast.

In any animal in which there is no distinct nervous system, it is obvious

that the general surface of the body must be sensitive to the action of its

surroundings, or to what are technically called stimuli. We know

experimentally that this is so in the case of the Protozoa, and of some

very simple Metazoa, such as the freshwater Polype or Hydra, where there is

no distinct nervous system. The skin or epidermis of the ancestor of the

Metazoa was no doubt similarly sensitive; and the fact of the nervous

system being derived from the epiblast implies that the functions of the

central nervous system, which were originally taken by the whole skin,

became gradually concentrated in a special part of the skin which was step

by step removed from the surface, and finally became a well-defined organ

in the interior of the body.

What were the steps by which this remarkable process took place? How has it

come about that there are nerves passing from the central nervous system to

all parts of the skin, and also to the muscles? How have the arrangements

for reflex actions arisen by which stimuli received on the surface of the

body are carried to the central part of the nervous system, and are thence

transmitted to the appropriate muscles, and cause them to contract? All

these questions require to be answered before we can be said to possess a

satisfactory knowledge of the origin of the nervous system. As yet,

however, the knowledge of these points derived from embryology is

imperfect, although there is every hope that further investigation will

render it less so. Fortunately, however, a study of comparative anatomy,

especially that of the Coelenterata, fills up some of the gaps left from

our study of embryology.

From embryology we learn that the ganglion-cells of the central part of the

nervous system are originally derived from the simple undifferentiated

epithelial cells of the surface of the body. We further learn that the

nerves are out-growths of the central nervous system. It was supposed till

quite recently that the nerves in Vertebrates were derived from parts of

the middle germinal layer or mesoblast, and that they only became

secondarily connected with the central nervous system. This is now known

not to be the case, but the nerves are formed as processes growing out from

the central part of the nervous system.

Another important fact shewn by embryology is that the central nervous

system, and percipient portion of the organs of special sense, are often

formed from the same part of the primitive epidermis. Thus, in ourselves

and in other vertebrate animals the sensitive part of the eye, known as the

retina, is formed from two lateral lobes of the front part of the primitive

brain. The crystalline lens and cornea of the eye are, however,

subsequently formed from the skin.

The same is true for the peculiar compound eyes of crabs or Crustacea. The

most important part of the central nervous system of these animals is the

supra-oesophageal ganglia, often known as the brain, and these are formed

in the embryo from two thickened patches of the skin at the front end of

the body. These thickened patches become gradually detached from the

surface, remaining covered over by a layer of skin. They then constitute

the supra-oesophageal ganglia; but they form not only the ganglia, but also

the rhabdons or retinal elements of the eye--the parts in fact which

correspond to the rods and cones in our own retina. The layer of epidermis

or skin which lies immediately above the supra-oesophageal ganglia becomes

gradually converted into the refractive media of the crustacean eye. A

cuticle which lies on its surface forms the peculiar facets on the surface

of the eye, which are known as the corneal lenses, while the cells of the

epidermis give rise to lens-like bodies known as the crystalline cones.

It would be easy to quote further instances of the same kind, but I trust

that the two which I have given will be sufficient to shew the kind of

relation which often exists between the organs of special sense, especially

those of vision, and the central nervous system. It might have been

anticipated _à priori_ that organs of special sense would only appear in

animals provided with a well-developed central nervous system. This,

however, is not the case. Special cells, with long delicate hairs, which

are undoubtedly highly sensitive structures, are present in animals in

which as yet nothing has been found which could be called a central nervous

system; and there is every reason to think that the organs of special sense

originated _pari passu_ with the central nervous system. It is probable

that in the simplest organisms the whole body is sensitive to light, but

that with the appearance of pigment-cells in certain parts of the body, the

sensitiveness to light became localised to the areas where the

pigment-cells were present. Since, however, it was necessary that stimuli

received by such organs should be communicated to other parts of the body,

some of the epidermic cells in the neighbourhood of the pigment-spots,

which were at first only sensitive, in the same manner as other cells of

the epidermis, became gradually differentiated into special nerve-cells. As

to the details of this differentiation, embryology does not as yet throw

any great light; but from the study of comparative anatomy there are

grounds for thinking that it was somewhat as follows:--Cells placed on the

surface sent protoplasmic processes of a nervous nature inwards, which came

into connection with nervous processes from similar cells placed in other

parts of the body. The cells with such processes then became removed from

the surface, forming a deeper layer of the epidermis below the sensitive

cells of the organ of vision. With these cells they remained connected by

protoplasmic filaments, and thus they came to form a thickening of the

epidermis underneath the organ of vision, the cells of which received their

stimuli from those of the organ of vision, and transmitted the stimuli so

received to other parts of the body. Such a thickening would obviously be

the rudiment of a central nervous system, and it is easy to see by what

steps it might become gradually larger and more important, and might

gradually travel inwards, remaining connected with the sense organ at the

surface by protoplasmic filaments, which would then constitute nerves. The

rudimentary eye would at first merely consist partly of cells sensitive to

light, and partly of optical structures constituting the lens, which would

throw an image of external objects upon it, and so convert the whole

structure into a true organ of vision. It has thus come about that, in the

development of the individual, the retina or sensitive part of the eye is

first formed in connection with the central nervous system, while the

lenses of the eye are independently evolved from the epidermis at a later

period.

The general features of the origin of the nervous system which have so far

been made out by means of the study of embryology are the following:--

(1) That the nervous system of the higher Metazoa has been developed in the

course of a long series of generations by a gradual process of

differentiation of parts of the epidermis.

(2) That part of the central nervous system of many forms arose as a local

collection of nerve-cells in the epidermis, in the neighbourhood of

rudimentary organs of vision.

(3) That ganglion cells have been evolved from simple epithelial cells of

the epidermis.

(4) That the primitive nerves were outgrowths of the original ganglion

cells; and that the nerves of the higher forms are formed as outgrowths of

the central nervous system.

The points on which embryology has not yet thrown a satisfactory light

are:--

(1) The steps by which the protoplasmic processes, from the primitive

epidermic cells, became united together so as to form a network of

nerve-fibres, placing the various parts of the body in nervous

communication.

(2) The process by which nerves became connected with muscles, so that a

stimulus received by a nerve-cell could be communicated to and cause a

contraction in a muscle.

Recent investigations on the anatomy of the Coelenterata, especially of

jelly-fish and sea-anemones, have thrown some light on these points,

although there is left much that is still obscure.

In our own country Mr Romaines has conducted some interesting physiological

experiments on these forms; and Professor Schäfer has made some important

histological investigations upon them. In Germany a series of interesting

researches have also been made on them by Professors Kleinenberg, Claus and

Eimer, and more especially by the brothers Hertwig, of Jena. Careful

histological investigations, especially those of the last-named authors,

have made us acquainted with the forms of some very primitive types of

nervous system. In the common sea-anemones there are, for instance, no

organs of special sense, and no definite central nervous system. There are,

however, scattered throughout the skin, and also throughout the lining of

the digestive tract, a number of specially modified epithelial cells, which

are no doubt delicate organs of sense. They are provided at their free

extremity with a long hair, and are prolonged on their inner side into a

fine process which penetrates the deeper part of the epithelial layer of

the skin or digestive wall. They eventually join a fine network of

protoplasmic fibres which forms a special layer immediately within the

epithelium. The fibres of this network are no doubt essentially nervous. In

addition to fibres there are, moreover, present in the network cells of the

same character as the multipolar ganglion-cells in the nervous system of

Vertebrates, and some of these cells are characterized by sending a process

into the superjacent epithelium. Such cells are obviously epithelial cells

in the act of becoming nerve-cells; and it is probable that the nerve-cells

are, in fact, sense-cells which have travelled inwards and lost their

epithelial character.

There is every reason to think that the network just described is not only

continuous with the sense-cells in the epithelium, but that it is also

continuous with epithelial cells which are provided with muscular

prolongations. The nervous system thus consists of a network of

protoplasmic fibres, continuous on the one hand with sense-cells in the

epithelium, and on the other with muscular cells. The nervous network is

generally distributed both beneath the epithelium of the skin and that of

the digestive tract, but is especially concentrated in the disc-like region

between the mouth and tentacles. The above observations have thrown a very

clear light on the characters of the nervous system at an early stage of

its evolution, but they leave unanswered the questions (1) how the nervous

network first arose, and (2) how its fibres became continuous with muscles.

It is probable that the nervous network took its origin from processes of

the sense-cells. The processes of the different cells probably first met

and then fused together, and, becoming more arborescent, finally gave rise

to a complicated network.

The connection between this network and the muscular cells also probably

took place by a process of contact and fusion.

Epithelial cells with muscular processes were discovered by Kleinenberg

before epithelial cells with nervous processes were known, and he suggested

that the epithelial part of such cells was a sense-organ, and that the

connecting part between this and the contractile processes was a

rudimentary nerve. This ingenious theory explained completely the fact of

nerves being continuous with muscles; but on the further discoveries being

made which I have just described, it became obvious that this theory would

have to be abandoned, and that some other explanation would have to be

given of the continuity between nerves and muscles. The hypothetical

explanation just offered is that of fusion.

It seems very probable that many of the epithelial cells were originally

provided with processes the protoplasm of which, like that of the Protozoa,

carried on the functions of nerves and muscles at the same time, and that

these processes united amongst themselves into a network. By a process of

differentiation parts of this network may have become specially

contractile, and other parts may have lost their contractility and become

solely nervous. In this way the connection between nerves and muscles might

be explained, and this hypothesis fits in very well with the condition of

the neuro-muscular system as we find it in the Coelenterata.

The nervous system of the higher Metazoa appears then to have originated

from a differentiation of some of the superficial epithelial cells of the

body, though it is possible that some parts of the system may have been

formed by a differentiation of the alimentary epithelium. The cells of the

epithelium were most likely at the same time contractile and sensory, and

the differentiation of the nervous system may very probably have commenced,

in the first instance, from a specialization in the function of part of a

network formed of neuro-muscular prolongations of epithelial cells. A

simultaneous differentiation of other parts of the network into muscular

fibres may have led to the continuity at present obtaining between nerves

and muscles.

Local differentiations of the nervous network, which was no doubt

distributed over the whole body, took place on the formation of organs of

special sense, and such differentiations gave rise to the formation of a

central nervous system. The central nervous system was at first continuous

with the epidermis, but became separated from it and travelled inwards.

Ganglion-cells took their origin from sensory epithelial cells, provided

with prolongations, continuous with the nervous network. Such epithelial

cells gradually lost their epithelial character, and finally became

completely detached from the epidermis.

Nerves, such as we find them in the higher types, originated from special

differentiations of the nervous network, radiating from the parts of the

central nervous system.

Such, briefly, is the present state of our knowledge as to the genesis of

the nervous system. I ought not, however, to leave this subject without

saying a few words as to the hypothetical views which the distinguished

evolutionist Mr Herbert Spencer has put forward on this subject in his work

on Psychology.

For Herbert Spencer nerves have originated, not as processes of epithelial

cells, but from the passage of motion along the lines of least resistance.

The nerves would seem, according to this view, to have been formed in any

tissue from the continuous passage of nervous impulses through it. "A wave

of molecular disturbance," he says, "passing along a tract of mingled

colloids closely allied in composition, and isomerically transforming the

molecules of one of them, will be apt at the same time to form some new

molecules of the same type," and thus a nerve becomes established.

A nervous centre is formed, according to Herbert Spencer, at the point in

the colloid in which nerves are generated, where a single nervous wave

breaks up, and its parts diverge along various lines of least resistance.

At such points some of the nerve-colloid will remain in an amorphous state,

and as the wave of molecular motion will there be checked, it will tend to

cause decompositions amongst the unarranged molecules. The decompositions

must, he says, cause "additional molecular motion to be disengaged; so that

along the outgoing lines there will be discharged an augmented wave. Thus

there will arise at this point something having the character of a ganglion

corpuscle."

These hypotheses of Herbert Spencer, which have been widely adopted in this

country, are, it appears to me, not borne out by the discoveries to which I

have called your attention to-day. The discovery that nerves have been

developed from processes of epithelial cells, gives a very different

conception of their genesis to that of Herbert Spencer, which makes them

originate from the passage of nervous impulses through a tract of mingled

colloids; while the demonstration that ganglion-cells arose as epithelial

cells of special sense, which have travelled inwards from the surface,

admits still less of a reconciliation with Herbert Spencer's view on the

same subject.

Although the present state of our knowledge on the genesis of the nervous

system is a great advance on that of a few years ago, there is still much

remaining to be done to make it complete.

The subject is well worth the attention of the morphologist, the

physiologist, or even of the psychologist, and we must not remain satisfied

by filling up the gaps in our knowledge by such hypotheses as I have been

compelled to frame. New methods of research will probably be required to

grapple with the problems that are still unsolved; but when we look back

and survey what has been done in the past, there can be no reason for

mistrusting our advance in the future.

XX. ON THE DEVELOPMENT OF THE SKELETON OF THE PAIRED FINS OF

ELASMOBRANCHII, CONSIDERED IN RELATION TO ITS BEARINGS ON THE NATURE OF

THE LIMBS OF THE VERTEBRATA[479].

Footnote 479: From the _Proceedings of the Zoological Society

of London_, 1881.

(With Plate 33.)

Some years ago the study of the development of the soft parts of the fins

in several Elasmobranch types, more especially in _Torpedo,_ led me to the

conclusion that the vertebrate limbs were remnants of two continuous

lateral fins[480]. More or less similar views (which I was not at that time

acquainted with) had been previously held by Maclise, Humphrey, and other

anatomists; these views had not, however, met with much acceptance, and

diverge in very important points from those put forward by me. Shortly

after the appearance of my paper, J. Thacker published two interesting

memoirs comparing the skeletal parts of the paired and unpaired fins[481].

Footnote 480: "Monograph on the Development of Elasmobranch

Fishes," pp. 319, 320.

Footnote 481: J. K. Thacker, "Median and Paired Fins; a

Contribution to the History of the Vertebrate Limbs," _Trans.

of the Connecticut Acad._ Vol. III. 1877. "Ventral Fins of

Ganoids," _Trans. of the Connecticut Acad._ Vol. IV. 1877.

In these memoirs Thacker arrives at conclusions as to the nature of the

fins in the main similar to mine, but on entirely independent grounds. He

attempts to shew that the structure of the skeleton of the paired fins is

essentially the same as that of the unpaired fins, and in this comparison

lays special stress on the very simple skeleton of the pelvic fin in the

cartilaginous Ganoids, more especially in _Acipenser_ and _Polyodon_. He

points out that the skeleton of the pelvic fin of _Polyodon_ consists

essentially of a series of nearly isolated rays, which have a strikingly

similar arrangement to that of the rays of the skeleton in many unpaired

fins. He sums up his views in the following way[482]:--

Footnote 482: _Loc. cit._ p. 298.

"As the dorsal and anal fins were specializations of the median folds

of _Amphioxus,_ so the paired fins were specializations of the two

lateral folds which are supplementary to the median in completing the

circuit of the body. These lateral folds, then, are the homologues of

Wolffian ridges, in embryos of higher forms. Here, as in the median

fins, there were formed chondroid and finally cartilaginous rods.

These became at least twice segmented. The orad ones, with more or

less concrescence proximally, were prolonged inwards. The cartilages

spreading met in the middle line; and a later extension of the

cartilages dorsad completed the limb-girdle.

"The limbs of the Protognathostomi consisted of a series of parallel

articulated cartilaginous rays. They may have coalesced somewhat

proximally and orad. In the ventral pair they had extended themselves

mesiad until they had nearly or quite met and formed the hip-girdle;

they had not here extended themselves dorsad. In the pectoral limb the

same state of things prevailed, but was carried a step further,

namely, by the dorsal extension of the cartilage constituting the

scapular portion, thus more nearly forming a ring or girdle."

The most important point in Thacker's theories which I cannot accept is the

derivation of the folds, of which the paired fins of the Vertebrata are

supposed to be specializations, from the lateral folds of _Amphioxus;_ and

Thacker himself recognizes that this part of his theory stands on quite a

different footing to the remainder.

Not long after the publication of Thacker's paper, an important memoir was

published by Mivart in the _Transactions_ of this Society[483]. The object

of the researches recorded in this paper was, as Mivart explains, to test

how far the hard parts of the limbs and of the azygos fins may have arisen

through centripetal chondrifications or calcifications, and so be

genetically exoskeletal[484].

Footnote 483: St George Mivart, "On the Fins of

Elasmobranchii," _Zoological Trans._ Vol. X.

Footnote 484: Mivart used the term exoskeletal in an unusual

and (as it appears to me) inconvenient manner. The term is

usually applied to dermal skeletal structures; but the skeleton

of the limbs, with which we are here concerned, is undoubtedly

not of this nature.

Mivart's investigations and the majority of his views were independent of

Thacker's memoir; but he acknowledges that he has derived from Thacker the

view that pelvic and pectoral girdles, as well as the skeleton of the

limbs, may have arisen independently of the axial skeleton.

The descriptive part of Mivart's paper contains an account of the structure

of a great variety of interesting and undescribed types of paired and

unpaired fins, mainly of Elasmobranchii. The following is the summary given

by Mivart of the conclusions at which he has arrived[485]:--

Footnote 485: _Loc. cit._ p. 480.

"1. Two continuous lateral longitudinal folds were developed, similar

to dorsal and ventral median longitudinal folds.

"2. Separate narrow solid supports (radials), in longitudinal series,

and with their long axes directed more or less outwards at right

angles with the long axis of the body, were developed in varying

extents in all these four longitudinal folds.

"3. The longitudinal folds became interrupted variously, but so as to

form two prominences on each side, _i.e._ the primitive paired limbs.

"4. Each anterior paired limb increased in size more rapidly than the

posterior limb.

"5. The bases of the cartilaginous supports coalesced as was needed,

according to the respective practical needs of the different separate

portions of the longitudinal folds, _i.e._ the respective needs of the

several fins.

"6. Occasionally the dorsal radials coalesced (as in _Notidanus_, &c.)

and sought centripetally (_Pristis_, &c.) adherence to the skeletal

axis.

"7. The radials of the hinder paired limb did so more constantly, and

ultimately prolonged themselves inwards by mesiad growth from their

coalesced base, till the piscine pelvic structure arose, as, _e.g._,

in _Squatina_.

"8. The pectoral radials with increasing development also coalesced

proximally, and thence prolonging themselves inwards to seek a _point

d'appui_, shot dorsad and ventrad to obtain a firm support, and at the

same time to avoid the visceral cavity. Thus they came to abut

dorsally against the axial skeleton, and to meet ventrally together in

the middle line below.

"9. The lateral fins, as they were applied to support the body on the

ground, became elongated, segmented, and narrowed, so that probably

the line of the propterygium, or possibly that of the mesopterygium,

became the cheiropterygial axis.

"10. The distal end of the incipient cheiropterygium either preserved

and enlarged preexisting cartilages or developed fresh ones to serve

fresh needs, and so grew into the developed cheiropterygium; but there

is not yet enough evidence to determine what was the precise course of

this transformation.

"11. The pelvic limb acquired a solid connection with the axial

skeleton (a pelvic girdle) through its need of a _point d'appui_ as a

locomotive organ on land.

"12. The pelvic limb became also elongated; and when its function was

quite similar to that of the pectoral limb, its structure became also

quite similar (_e.g. Ichthyosaurus_, _Plesiosaurus_, _Chelydra_, &c.);

but for the ordinary quadrupedal mode of progression it became

segmented and inflected in a way generally parallel with, but (from

its mode of use) in part inversely to, the inflections of the pectoral

limb."

Günther[486] has propounded a theory on the primitive character of the

fins, which, on the whole, fits in with the view that the paired fins are

structures of the same nature as the unpaired fins. The interest of

Günther's views on the nature of the skeleton of the fins more especially

depends upon the fact that he attempts to evolve the fin of _Ceratodus_

from the typical Selachian type of pectoral fin. His own statement on this

subject is as follows[487]:--

Footnote 486: "Description of _Ceratodus_," _Phil. Trans._

1871.

Footnote 487: _Loc. cit._ p. 534.

"On further inquiry into the more distant relations of the

_Ceratodus_-limb, we may perhaps be justified in recognizing in it a

modification of the typical form of the Selachian pectoral fin.

Leaving aside the usual treble division of the carpal cartilage

(which, indeed, is sometimes simple), we find that this shovel-like

carpal forms the base for a great number of phalanges, which are

arranged in more or less regular transverse rows (zones) and in

longitudinal rows (series). The number of phalanges of the zones and

series varies according to the species and the form of the fin; in

_Cestracion philippi_ the greater number of phalanges is found in the

proximal zones and middle series, all the phalanges decreasing in size

from the base of the fin towards the margins. In a Selachian with a

long, pointed, scythe-shaped pectoral fin, like that of _Ceratodus_,

we may, from analogy, presume that the arrangement of the cartilages

might be somewhat like that shewn in the accompanying diagram, which I

have divided into nine zones and fifteen series.

"When we now detach the outermost phalanx from each side of the first

horizontal zone, and with it the other phalanges of the same series,

when we allow the remaining phalanges of this zone to coalesce into

one piece (as, in nature, we find coalesced the carpals of _Ceratodus_

and many phalanges in Selachian fins), and when we repeat this same

process with the following zones and outer series, we arrive at an

arrangement identical with what we actually find in _Ceratodus_."

While the researches of Thacker and Mivart are strongly confirmatory of the

view at which I had arrived with reference to the nature of the paired

fins, other hypotheses as to the nature of the skeleton of the fins have

been enunciated, both before and after the publication of my memoir, which

are either directly or indirectly opposed to my view.

Huxley in his memoir on _Ceratodus_, which throws light on so many

important morphological problems, has dealt with the nature of paired

fins[488].

Footnote 488: T. H. Huxley, "On _Ceratodus Fosteri_, with some

Observations on the Classification of Fishes," _Proc. Zool.

Soc._ 1876.

He holds, in accordance with a view previously adopted by Gegenbaur, that

the limb of _Ceratodus_ "presents us with the nearest known approximation

to the fundamental form of vertebrate limb or archipterygium," and is of

opinion that in a still more archaic fish than _Ceratodus_ the skeleton of

the fin "would be made up of homologous segments, which might be termed

pteromeres, each of which would consist of a mesomere with a preaxial and a

postaxial paramere." He considers that the pectoral fins of Elasmobranchii,

more especially the fin of _Notidanus_, which he holds to be the most

primitive form of Elasmobranch fin, "results in the simplest possible

manner from the shortening of the axis of such a fin-skeleton as that of

_Ceratodus_, and the coalescence of some of its elements." Huxley does not

enter into the question of the origin of the skeleton of the pelvic fin of

Elasmobranchii.

It will be seen that Huxley's idea of the primitive structure of the

archipterygium is not easily reconcilable with the view that the paired

fins are parts of a once continuous lateral fin, in that the skeleton of

such a lateral fin, if it has existed, must necessarily have consisted of a

series of parallel rays.

Gegenbaur[489] has done more than any other living anatomist to elucidate

the nature of the fins; and his views on this subject have undergone

considerable changes in the course of his investigations. After Günther had

worked out the structure of the fin of _Ceratodus_, Gegenbaur suggested

that it constituted the most primitive _persisting_ type of fin, and has

moreover formed a theory as to the origin of the fins founded on this view,

to the effect that the fins, together with their respective girdles, are to

be derived from visceral arches with their rays.

Footnote 489: C. Gegenbaur, _Untersuchungen z. vergleich.

Anat. d. Wirbelthiere_ (Leipzig 1864-5): erstes Heft, "Carpus

u. Tarsus;" zweites Heft, "Brustflosse d. Fische." "Ueb. d.

Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d.

Hintergliedmaassen d. Selachier insbesondere," _Jenaische

Zeitschrift_, Vol. V. 1870. "Ueb. d. Archipterygium,"

_Jenaische Zeitschrift_, Vol. VII. 1873. "Zur Morphologie d.

Gliedmaassen d. Wirbelthiere," _Morphologisches Jahrbuch_, Vol. II.

1876.

His views on this subject are clearly explained in the subjoined passages

quoted from the English translation of his _Elements of Comparative

Anatomy_, pp. 473 and 477.

"The skeleton of the free appendage is attached to the extremity of

the girdle. When simplest, this is made up of cartilaginous rods

(rays), which differ in their size, segmentation, and relation to one

another. One of these rays is larger than the rest, and has a number

of other rays attached to its sides. I have given the name of

_archipterygium_ to the ground-form of the skeleton which extends from

the limb-bearing girdle into the free appendage. The primary ray is

the stem of this archipterygium, the characters of which enable us to

follow out the lines of development of the skeleton of the appendage.

Cartilaginous arches beset with the rays form the branchial skeleton.

The form of skeleton of the appendages may be compared with them; and

we are led to the conclusion that it is possible that they may have

been derived from such forms. In the branchial skeleton of the

Selachii the cartilaginous bars are beset with simple rays. In many a

median one is developed to a greater size. As the surrounding rays

become smaller, and approach the larger one, we get an intermediate

step towards that arrangement in which the larger median ray carries a

few smaller ones. This differentiation of one ray, which is thereby

raised to a higher grade, may be connected with the primitive form of

the appendicular skeleton; and as we compare the girdle with a

branchial arch, so we may compare the median ray and its secondary

investment of rays with the skeleton of the free appendage.

"All the varied forms which the skeleton of the free appendages

exhibits may be derived from a ground-form which persists in a few

cases only, and which represents the first, and consequently the

lowest, stage of the skeleton in the fin--the _archipterygium_. This

is made up of a stem which consists of jointed pieces of cartilage,

which is articulated to the shoulder-girdle and is beset on either

side with rays which are likewise jointed. In addition to the rays of

the stem there are others which are directly attached to the

limb-girdle.

"_Ceratodus_ has a fin-skeleton of this form; in it there is a stem

beset with two rows of rays. But there are no rays in the

shoulder-girdle. This biserial investment of rays on the stem of the

fin may also undergo various kinds of modifications. Among the Dipnoi,

_Protopterus_ retains the medial row of rays only, which have the form

of fine rods of cartilage; in the Selachii, on the other hand, the

lateral rays are considerably developed. The remains of the medial row

are ordinarily quite small, but they are always sufficiently distinct

to justify us in supposing that in higher forms the two sets of rays

might be better developed. Rays are still attached to the stem and are

connected with the shoulder-girdle by means of larger plates. The

joints of the rays are sometimes broken up into polygonal plates which

may further fuse with one another; concrescence of this kind may also

affect the pieces which form the base of the fin. By regarding the

free rays, which are attached to these basal pieces, as belonging to

these basal portions, we are able to divide the entire skeleton of the

fin into three segments--pro-, meso-, and metapterygium.

"The metapterygium represents the stem of the archipterygium and the

rays on it. The propterygium and the mesopterygium are evidently

derived from the rays which still remain attached to the

shoulder-girdle."

Since the publication of the memoirs of Thacker, Mivart, and myself, a

pupil of Gegenbaur's, M. v. Davidoff[490], has made a series of very

valuable observations, in part directed towards demonstrating the

incorrectness of our theoretical views, more especially Thacker's and

Mivart's view of the genesis of the skeleton of the limbs. Gegenbaur[491]

has also written a short paper in connection with Davidoff's memoir, in

support of his own as against our views.

Footnote 490: M. v. Davidoff, "Beiträge z. vergleich. Anat. d.

hinteren Gliedmaassen d. Fische, I.," _Morphol. Jahrbuch_,

Vol. V. 1879.

Footnote 491: "Zur Gliedmaassenfrage. An die Untersuchungen

von Davidoff's angeknüpfte Bemerkungen," _Morphol. Jahrbuch_,

Vol. V. 1879.

It would not be possible here to give an adequate account of Davidoff's

observations on the skeleton, muscular system, and nerves of the pelvic

fins. His main argument against the view that the paired fins are the

remains of a continuous lateral fin is based on the fact that a variable

but often considerable number of the spinal nerves in front of the pelvic

fin are united by a longitudinal commissure with the true plexus of the

nerves supplying the fin. From this he concludes that the pelvic fin has

shifted its position, and that it may once therefore have been situated

close behind the visceral arches. Granting, however, that Davidoff's

deduction from the character of the pelvic plexus is correct, there is, so

far as I see, no reason in the nature of the lateral-fin theory why the

pelvic fins should not have shifted; and, on the other hand, the

longitudinal cord connecting some of the ventral roots in front of the

pelvic fin may have another explanation. It may, for instance, be a remnant

of the time when the pelvic fin had a more elongated form than at present,

and accordingly extended further forwards.

In any case our knowledge of the nature and origin of nervous plexuses is

far too imperfect to found upon their characters such conclusions as those

of Davidoff.

Gegenbaur, in his paper above quoted, further urges against Thacker and

Mivart's views the fact that there is no proof that the fin of _Polyodon_

is a primitive type; and also suggests that the epithelial line which I

have found connecting the embryonic pelvic and pectoral fins in _Torpedo_

may be a rudiment indicating a migration backwards of the pelvic fin.

With reference to the development of the pectoral fin in the Teleostei

there are some observations of 'Swirski[492], which unfortunately do not

throw very much light upon the nature of the limb.

Footnote 492: G. 'Swirski, _Untersuch. üb. d. Entwick. d.

Schultergürtels u. d. Skelets d. Brustflosse d. Hechts._ Inaug.

Diss. Dorpat, 1880.

'Swirski finds that in the Pike the skeleton of the limb is formed of a

plate of cartilage continuous with the pectoral girdle, which soon becomes

divided into a proximal and a distal portion. The former is subsequently

segmented into five basal rays, and the latter into twelve parts, the

number of which subsequently becomes reduced.

* * * * *

The observations which I have to lay before the Society were made with the

object of determining how far the development of the skeleton of the limbs

throws light on the points on which the anatomists whose opinions have just

been quoted are at variance.

They were made, in the first instance, to complete a chapter in my work on

comparative embryology; and, partly owing to the press of other

engagements, but still more to the difficulty of procuring material, my

observations are confined to the two British species of the genus

_Scyllium_, viz. _Sc. stellare_ and _Sc. canicula_; yet I venture to

believe that the results at which I have arrived are not wholly without

interest.

Before dealing with the development of the skeleton of the fin, it will be

convenient to describe with great brevity the structure of the pectoral and

pelvic fins of the adult. The pectoral fins consist of broad plates

inserted horizontally on the sides of the body; so that in each there may

be distinguished a dorsal and a ventral surface, and an anterior and a

posterior border. Their shape may best be gathered from the woodcut

(fig. 1); and it is to be especially noted that the narrowest part of the

fin is the base, where it is attached to the side of the body. The

cartilaginous skeleton only occupies a small zone at the base of the fin,

the remainder being formed of a fringe supported by radiately arranged

horny fibres[493].

Footnote 493: The horny fibres are mesoblastic products; they

are formed, in the first instance, as extremely delicate

fibrils on the inner side of the membrane separating the

epiblast from the mesoblast.

[Illustration: FIG. 1.

Pectoral fins and girdle of an adult of _Scyllium canicula_ (natural size,

seen from behind and above).

_co._ Coracoid. _sc._ scapula. _pp._ propterygium. _mep._ mesopterygium.

_mp._ metapterygium. _fn._ part of fin supported by horny fibre.]

[Illustration: FIG. 2.

Right pelvic fin and part of pelvic girdle of an adult female of _Scyllium

canicula_ (natural size).

_il._ iliac process. _pn._ pubic process, cut across below. _bp._

basipterygium. _af._ anterior cartilaginous fin-ray articulated to pelvic

girdle. _fn._ part of fin supported by horny fibres.]

The true skeleton consists of three basal pieces articulating with the

pectoral girdle; on the outer side of which there is a series of more or

less segmented cartilaginous fin-rays. Of the basal cartilages one (_pp_)

is anterior, a second (_mep_) is placed in the middle, and a third is

posterior (_mp_). They have been named by Gegenbaur the _propterygium_, the

_mesopterygium_, and the _metapterygium_; and these names are now generally

adopted.

The metapterygium is by far the most important of the three, and in

_Scyllium canicula_ supports 12 or 13 rays[494]. It forms a large part of

the posterior boundary of the fin, and bears rays only on its _anterior_

border.

Footnote 494: In one example where the metapterygium had 13

rays the mesopterygium had only 2 rays.

The mesopterygium supports 2 or 3 rays, in the basal parts of which the

segmentation into distinct rays is imperfect; and the propterygium supports

only a single ray.

The pelvic fins are horizontally placed, like the pectoral fins, but differ

from the latter in nearly meeting each other along the median ventral line

of the body. They also differ from the pectoral fins in having a relatively

much broader base of attachment to the sides of the body. Their

cartilaginous skeleton (woodcut, fig. 2) consists of a basal bar, placed

parallel to the base of the fin, and articulated in front with the pelvic

girdle.

On its outer border it articulates with a series of cartilaginous fin-rays.

I shall call the basal bar the basipterygium. The rays which it bears are

most of them less segmented than those of the pectoral fin, being only

divided into two; and the posterior ray, which is placed in the free

posterior border of the fin, continues the axis of the basipterygium. In

the male it is modified in connection with the so-called clasper.

The anterior fin-ray of the pelvic fin, which is broader than the other

rays, articulates directly with the pelvic girdle, instead of with the

basipterygium. This ray, in the female of _Scyllium canicula_ and in the

male of _Scyllium catulus_ (Gegenbaur), is peculiar in the fact that its

distal segment is longitudinally divided into two or more pieces, instead

of being single as is the case with the remaining rays. It is probably

equivalent to two of the posterior rays.

_Development of the paired Fins._--The first rudiments of the limbs appear

in _Scyllium_, as in other fishes, as slight longitudinal ridge-like

thickenings of the epiblast, which closely resemble the first rudiments of

the unpaired fins.

These ridges are two in number on each side--an anterior immediately behind

the last visceral fold, and a posterior on the level of the cloaca. In most

Fishes they are in no way connected; but in some Elasmobranch embryos, more

especially in that of _Torpedo_, they are connected together at their first

development by a line of columnar epiblast cells. This connecting line of

columnar epiblast, however, is a very transitory structure. The rudimentary

fins soon become more prominent, consisting of a projecting ridge both of

epiblast and mesoblast, at the outer edge of which is a fold of epiblast

only, which soon reaches considerable dimensions. At a later stage the

mesoblast penetrates into this fold, and the fin becomes a simple ridge of

mesoblast covered by epiblast. The pectoral fins are at first considerably

ahead of the pelvic fins in development.

The direction of the original epithelial line which connected the two fins

of each side is nearly, though not quite, longitudinal, sloping somewhat

obliquely ventralwards. It thus comes about that the attachment of each

pair of limbs is somewhat on a slant, and that the pelvic pair nearly meet

each other in the median ventral line shortly behind the anus.

The embryonic muscle-plates, as I have elsewhere shewn, grow into the bases

of the fins; and the cells derived from these ingrowths, which are placed

on the dorsal and ventral surfaces in immediate contact with the epiblast,

probably give rise to the dorsal and ventral muscular layers of the limb,

which are shewn in section in Plate 33, fig. 1, _m_, and in Plate 33,

fig. 7, _m_.

The cartilaginous skeleton of the limbs is developed in the indifferent

mesoblast cells between the two layers of muscles. Its early development in

both the pectoral and the pelvic fins is very similar. When first visible

it differs histologically from the adjacent mesoblast simply in the fact of

its cells being more concentrated; while its boundary is not sharply

marked.

At this stage it can only be studied by means of sections. It arises

simultaneously and continuously with the pectoral and pelvic girdles, and

consists, in both fins, of a bar springing at right angles from the

posterior side of the pectoral or pelvic girdle, and running parallel to

the long axis of the body along the base of the fin. The outer side of this

bar is continued into a thin plate, which extends into the fin.

The structure of the skeleton of the fin slightly after its first

differentiation will be best understood from Plate 33, fig. 1, and Plate

33, fig. 7. These figures represent transverse sections through the pelvic

and pectoral fins of the same embryo on the same scale. The basal bar is

seen at _bp_, and the plate at this stage (which is considerably later than

the first differentiation) already partially segmented into rays at _br_.

Outside the region of the cartilaginous plate is seen the fringe with the

horny fibres (_h.f._); and dorsally and ventrally to the cartilaginous

skeleton are seen the already well-differentiated muscles (_m_).

The pectoral fin is shewn in horizontal section in Plate 33, fig. 6, at a

somewhat earlier stage than that to which the transverse sections belong.

The pectoral girdle (_p.g._) is cut transversely, and is seen to be

perfectly continuous with the basal bar (_vp_) of the fin. A similar

continuity between the basal bar of the pelvic fin and the pelvic girdle is

shewn in Plate 33, fig. 2, at a somewhat later stage. The plate continuous

with the basal bar of the fin is at first, to a considerable extent in the

pectoral, and to some extent in the pelvic fin, a continuous lamina, which

subsequently segments into rays. In the parts of the plate which eventually

form distinct rays, however, almost from the first the cells are more

concentrated than in those parts which will form the tissue between the

rays; and I am not inclined to lay any stress whatever upon the fact of the

cartilaginous fin-rays being primitively part of a continuous lamina, but

regard it as a secondary phenomenon, dependent on the mode of conversion of

embryonic mesoblast cells into cartilage. In all cases the separation into

distinct rays is to a large extent completed before the tissue of which the

plates are formed is sufficiently differentiated to be called cartilage by

an histologist.

The general position of the fins in relation to the body, and their

relative sizes, may be gathered from Plate 33, figs. 4 and 5, which

represent transverse sections of the same embryo as that from which the

transverse sections shewing the fin on a larger scale were taken.

During the first stage of its development the skeleton of both fins may

thus be described as consisting of _a longitudinal bar running along the

base of the fin, and giving off at right angles series of rays which pass

into the fin_. The longitudinal bar may be called the basipterygium; and it

is continuous in front with the pectoral or pelvic girdle, as the case may

be.

The further development of the primitive skeleton is different in the case

of the two fins.

_The Pelvic Fin._--The changes in the pelvic fin are comparatively slight.

Plate 33, fig. 2, is a representation of the fin and its skeleton in a

female of _Scyllium stellare_ shortly after the primitive tissue is

converted into cartilage, but while it is still so soft as to require the

very greatest care in dissection. The fin itself forms a simple projection

of the side of the body. The skeleton consists of a basipterygium (_bp_),

continuous in front with the pelvic girdle. To the outer side of the

basipterygium a series of cartilaginous fin-rays are attached--the

posterior ray forming a direct prolongation of the basipterygium, while the

anterior ray is united rather with the pelvic girdle than with the

basipterygium. All the cartilaginous fin-rays except the first are

completely continuous with the basipterygium, their structure in section

being hardly different from that shewn in Plate 33, fig. 1.

The external form of the fin does not change very greatly in the course of

the further development; but the hinder part of the attached border is, to

some extent, separated off from the wall of the body, and becomes the

posterior border of the adult fin. With the exception of a certain amount

of segmentation in the rays, the character of the skeleton remains almost

as in the embryo. The changes which take place are illustrated by Plate 33,

fig. 3, shewing the fin of a young male of _Scyllium stellare_. The

basipterygium has become somewhat thicker, but is still continuous in front

with the pelvic girdle, and otherwise retains its earlier characters. The

cartilaginous fin-rays have now become segmented off from it and from the

pelvic girdle, the posterior end of the basipterygial bar being segmented

off as the terminal ray.

The anterior ray is directly articulated with the pelvic girdle, and the

remaining rays continue articulated with the basipterygium. Some of the

latter are partially segmented.

As may be gathered by comparing the figure of the fin at the stage just

described with that of the adult fin (woodcut, fig. 2), the remaining

changes are very slight. The most important is the segmentation of the

basipterygial bar from the pelvic girdle.

The pelvic fin thus retains in all essential points its primitive

structure.

_The Pectoral Fin._--The earliest stage of the pectoral fin differs, as I

have shewn, from that of the pelvic fin only in minor points (Pl. 33,

fig. 6). There is the same longitudinal or basipterygial bar (_bp_), to

which the fin-rays are attached, which is continuous in front with the

pectoral girdle (_pg_). The changes which take place in the course of the

further development, however, are very much more considerable in the case

of the pectoral than in that of the pelvic fin.

The most important change in the external form of the fin is caused by a

reduction in the length of its attachment to the body. At first (Pl. 33,

fig. 6), the base of the fin is as long as the greatest breadth of the fin;

but it gradually becomes shortened by being constricted off from the body

at its hinder end. In connection with this process the posterior end of the

basipterygial bar is gradually rotated outwards, its anterior end remaining

attached to the pectoral girdle. In this way this bar comes to form the

posterior border of the skeleton of the fin (Pl. 33, figs. 8 and 9),

constituting the metapterygium (_mp_). It becomes eventually segmented off

from the pectoral girdle, simply articulating with its hinder edge.

The plate of cartilage, which is continued outwards from the basipterygium,

or, as we may now call it, the metapterygium, into the fin, is not nearly

so completely divided up into fin-rays as the homologous part of the pelvic

fin; and this is especially the case with the basal part of the plate. This

basal part becomes, in fact, at first only divided into two parts (Pl. 33,

fig. 8)--a small anterior part at the front end (_me.p_), and a larger

posterior along the base of the metapterygium (_mp_); and these two parts

are not completely segmented from each other. The anterior part directly

joins the pectoral girdle at its base, resembling in this respect the

anterior fin-ray of the pelvic girdle. It constitutes the (at this stage

undivided) rudiment of the mesopterygium and propterygium of Gegenbaur. It

bears in my specimen of this age four fin-rays at its extremity, the

anterior not being well marked. The remaining fin-rays are prolongations

outwards of the edge of the plate continuous with the metapterygium. These

rays are at the stage figured more or less transversely segmented; but at

their outer edge they are united together by a nearly continuous rim of

cartilage. The spaces between the fin-rays are relatively considerably

larger than in the adult.

The further changes in the cartilages of the pectoral limb are,

morphologically speaking, not important, and are easily understood by

reference to Pl. 33, fig. 9 (representing the skeleton of the limb of a

nearly ripe embryo). The front end of the anterior basal cartilage becomes

segmented off as a propterygium (_pp_), bearing a single fin-ray, leaving

the remainder of the cartilage as a mesopterygium (_mes_). The remainder of

the now considerably segmented fin-rays are borne by the metapterygium.

* * * * *

_General Conclusions._--From the above observations, conclusions of a

positive kind may be drawn as to the primitive structure of the skeleton;

and the observations have also, it appears to me, important bearings on the

theories of my predecessors in this line of investigation.

The most obvious of the positive conclusions is to the effect that the

embryonic skeleton of the paired fins consists of a series of parallel rays

similar to those of the unpaired fins. These rays support the soft parts of

the fins, which have the form of a longitudinal ridge; and they are

continuous at their base with a longitudinal bar. This bar, from its

position at the base of the fin, can clearly never have been a median axis

with the rays on both sides. It becomes the basipterygium in the pelvic

fin, which retains its embryonic structure much more completely than the

pectoral fin; and the metapterygium in the pectoral fin. The metapterygium

of the pectoral fin is thus clearly homologous with the basipterygium of

the pelvic fin, as originally supposed by Gegenbaur, and as has since been

maintained by Mivart. The propterygium and mesopterygium are obviously

relatively _unimportant_ parts of the skeleton as compared with the

metapterygium.

My observations on the development of the skeleton of the fins certainly do

not of themselves demonstrate that the paired fins are remnants of a once

continuous lateral fin; but they support this view in that they shew the

primitive skeleton of the fins to have exactly the character which might

have been anticipated if the paired fins had originated from a continuous

lateral fin. The longitudinal bar of the paired fins is believed by both

Thacker and Mivart to be due to the coalescence of the bases of the

primitively independent rays of which they believe the fin to have been

originally composed. This view is probable enough in itself, and is

rendered more so by the fact, pointed out by Mivart, that a longitudinal

bar supporting the cartilaginous rays of unpaired fins is occasionally

formed; but there is no trace in the embryo Scylliums of the bar in

question being formed by the coalescence of rays, though the fact of its

being perfectly continuous with the bases of the fin-rays is somewhat in

favour of such coalescence.

Thacker and Mivart both hold that the pectoral and pelvic girdles are

developed by ventral and dorsal growths of the anterior end of the

longitudinal bar supporting the fin-rays.

There is, so far as I see, no theoretical objection to be taken to this

view; and the fact of the pectoral and pelvic girdles originating

continuously and long remaining united with the longitudinal bars of their

respective fins is in favour of it rather than the reverse. The same may be

said of the fact that the first part of each girdle to be formed is that in

the neighbourhood of the longitudinal bar (basipterygium) of the fin, the

dorsal and ventral prolongations being subsequent growths.

On the whole my observations do not throw much light on the theories of

Thacker and Mivart as to the genesis of the skeleton of the paired fin;

but, so far as they bear on the subject, they are distinctly favourable to

those theories.

The main results of my observations appear to me to be decidedly adverse to

the views recently put forward on the structure of the fin by Gegenbaur and

Huxley, both of whom, as stated above, consider the primitive type of fin

to be most nearly retained in _Ceratodus_, and to consist of a central

multisegmented axis with numerous lateral rays.

Gegenbaur derives the Elasmobranch pectoral fin from a form which he calls

the archipterygium, nearly like that of _Ceratodus_, with a median axis and

two rows of rays--but holds that in addition to the rays attached to the

median axis, which are alone found in _Ceratodus_, there were other rays

directly articulated to the shoulder-girdle. He considers that in the

Elasmobranch fin the majority of the lateral rays on the posterior (or

median according to his view of the position of the limb) side have become

aborted, and that the central axis is represented by the metapterygium;

while the pro- and mesopterygium and their rays are, he believes, derived

from those rays of the archipterygium which originally articulated directly

with the shoulder-girdle.

This view appears to me to be absolutely negatived by the facts of

development of the pectoral fin in _Scyllium_--not so much because the

pectoral fin in this form is necessarily to be regarded as primitive, but

because what Gegenbaur holds to be the primitive axis of the biserial fin

is demonstrated to be really the base, and it is only in the adult that it

is conceivable that a second set of lateral rays could have existed on the

posterior side of the metapterygium. If Gegenbaur's view were correct, we

should expect to find in the embryo, if anywhere, traces of the second set

of lateral rays; but the fact is that, as may easily be seen by an

inspection of figs. 6 and 7, such a second set of lateral rays could not

possibly have existed in a type of fin like that found in the embryo. With

this view of Gegenbaur's it appears to me that the theory held by this

anatomist to the effect that the limbs are modified gill-arches also falls,

in that his method of deriving the limbs from gill-arches ceases to be

admissible, while it is not easy to see how a limb, formed on the type of

the embryonic limb of Elasmobranchii, could be derived from a gill-arch

with its branchial rays.

Gegenbaur's older view, that the Elasmobranch fin retains a primitive

uniserial type, appears to me to be nearer the truth than his more recent

view on this subject; though I hold the fundamental point established by

the development of these parts in _Scyllium_ to be that the posterior

border of the adult Elasmobranch pectoral fin is the primitive

base-line,_i.e._line of attachment of the fin to the side of the body.

Huxley holds that the mesopterygium is the proximal piece of the axial

skeleton of the limb of _Ceratodus_, and derives the Elasmobranch fin from

that of _Ceratodus_ by the shortening of its axis and the coalescence of

some of its elements. The entirely secondary character of the

mesopterygium, and its total absence in the young embryo _Scyllium_, appear

to me as conclusive against Huxley's view as the character of the embryonic

fin is against that of Gegenbaur; and I should be much more inclined to

hold that the fin of _Ceratodus_ has been derived from a fin like that of

the Elasmobranchii by a series of steps similar to those which Huxley

supposes to have led to the establishment of the Elasmobranch fin, but in

exactly the reverse order.

There is one statement of Davidoff's which I cannot allow to pass without

challenge. In comparing the skeletons of the paired and unpaired fins he is

anxious to prove that the former are independent of the axial skeleton in

their origin and that the latter have been segmented from the axial

skeleton, and thus to shew that an homology between the two is impossible.

In support of his view he states[495] that he has satisfied himself, from

embryos of _Acanthias_ and _Scyllium_, that the rays of the unpaired fins

_are undoubtedly products of the segmentation of the dorsal and ventral

spinous processes_.

Footnote 495: _Loc. cit._ p. 514.

This statement is wholly unintelligible to me. From my examination of the

development of the first dorsal and the anal fins of _Scyllium_ I find that

their rays develop at a considerable distance from, and quite independently

of, the neural and hæmal arches, and that they are at an early stage of

development distinctly in a more advanced state of histological

differentiation than the neural and hæmal arches of the same region. I have

also found exactly the same in the embryos of _Lepidosteus_.

I have, in fact, no doubt that the skeleton of both the paired and the

unpaired fins of Elasmobranchii and _Lepidosteus_ is in its development

independent of the axial skeleton. The phylogenetic mode of origin of the

skeleton both of the paired and of the unpaired fins cannot, however, be

made out without further investigation.

EXPLANATION OF PLATE 33.[496]

Footnote 496: I employ here the same letters to indicate the

stages as in my "Monograph on Elasmobranch Fishes."

Fig. 1. Transverse section through the pelvic fin of an embryo of

_Scyllium_ belonging to stage P{1}, magnified 50 diameters. _bp._

basipterygium. _br._ fin ray. _m._ muscle. _hf._ horny fibres supporting

the peripheral part of the fin.

Fig. 2. Pelvic fin of a very young female embryo of _Scyllium stellare_,

magnified 16 diameters. _bp._ basipterygium. _pu._ pubic process of pelvic

girdle (cut across below). _il._ iliac process of pelvic girdle. _fo._

foramen.

Fig. 3. Pelvic fin of a young male embryo of _Scyllium stellare_, magnified

16 diameters. _bp._ basipterygium. _mo._ process of basipterygium continued

into clasper. _il._ iliac process of pelvic girdle. _pu._ pubic section of

pelvic girdle.

Fig. 4. Transverse section through the ventral part of the trunk of an

embryo _Scyllium_ of stage P, in the region of the pectoral fins, to shew

how the fins are attached to the body, magnified 18 diameters. _br._

cartilaginous fin-ray. _bp._ basipterygium. _m._ muscle of fin. _mp._

muscle-plate.

Fig. 5. Transverse section through the ventral part of the trunk of an

embryo _Scyllium_ of stage P, in the region of the pelvic fin, on the same

scale as fig. 4. _bp._ basipterygium. _br._ cartilaginous fin-rays. _m._

muscle of the fins. _mp._ muscle-plate.

Fig. 6. Pectoral fin of an embryo of _Scyllium canicula_, of a stage

between O and P, in longitudinal and horizontal section (the skeleton of

the fin was still in the condition of embryonic cartilage), magnified 36

diameters. _bp._ basipterygium (eventual metapterygium). _fr._

cartilaginous fin-rays. _pg._ pectoral girdle in transverse section. _fo._

foramen in pectoral girdle. _pe._ epithelium of peritoneal cavity.

Fig. 7. Transverse section through the pectoral fin of a _Scyllium_ embryo

of stage P, magnified 50 diameters. _bp._ basipterygium. _br._

cartilaginous fin-ray. _m._ muscle. _hf._ horny fibres.

Fig. 8. Pectoral fin of an embryo of _Scyllium stellare_, magnified 16

diameters. _mp._ metapterygium (basipterygium of earlier stage). _me.p._

rudiment of future pro- and mesopterygium. _sc._ cut surface of a scapular

process. _cr._ coracoid process. _fr._ foramen. _hf._ horny fibres.

Fig. 9. Skeleton of the pectoral fin and part of pectoral girdle of a

nearly ripe embryo of _Scyllium stellare_, magnified 10 diameters. _mp._

metapterygium. _mes._ mesopterygium. _pp._ propterygium. _cr._ coracoid

process.

XXI. ON THE EVOLUTION OF THE PLACENTA, AND ON THE POSSIBILITY OF EMPLOYING

THE CHARACTERS OF THE PLACENTA IN THE CLASSIFICATION OF THE MAMMALIA[497].

Footnote 497: From the _Proceedings of the Zoological Society

of London_, 1881.

From Owen's observations on the Marsupials it is clear that the yolk-sack

in this group plays an important (if not the most important) part, in

absorbing the maternal nutriment destined for the foetus. The fact that in

Marsupials both the yolk-sack and the allantois are concerned in rendering

the chorion vascular, makes it _à priori_ probable that this was also the

case in the primitive types of the Placentalia; and this deduction is

supported by the fact that in the Rodentia, Insectivora, and Cheiroptera

this peculiarity of the foetal membranes is actually found. In the

primitive Placentalia it is also probable that from the discoidal allantoic

region of the chorion simple foetal villi, like those of the Pig, projected

into uterine crypts; but it is not certain how far the umbilical region of

the chorion, which was no doubt vascular, may also have been villous. From

such a primitive type of foetal membranes divergencies in various

directions have given rise to the types of foetal membranes found at the

present day.

In a general way it may be laid down that variations in any direction which

tended to increase the absorbing capacities of the chorion would be

advantageous. There are two obvious ways in which this might be done, viz.

(1) by increasing the complexity of the foetal villi and maternal crypts

over a limited area, (2) by increasing the area of the part of the chorion

covered by the placental villi. Various combinations of the two processes

would also, of course, be advantageous.

The most fundamental change which has taken place in all the existing

Placentalia is the exclusion of the umbilical vesicle from any important

function in the nutrition of the foetus.