NERVOUS SYSTEM.
_Origin of the Nervous System._
One of the most important recent embryological discoveries is the fact
that the central nervous system, in all the Metazoa in which it is
fully established, is (with a few doubtful exceptions) derived from
the primitive epiblast[152]. As we have already seen that the epiblast
represents to a large extent the primitive epidermis, 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 has
finally become in the higher types a well-defined organ imbedded in
the subdermal tissues.
[152] Whether there is any part of it in many types not so
derived requires further investigation, now that it has been
shewn by the Hertwigs that part of the system develops from the
endoderm in some Coelenterata. O. Hertwig holds that part of it
has a mesoblastic origin in Sagitta, but his observations on this
point appear to me very inconclusive. It would be very
advantageous to investigate the origin of Auerbach's plexus in
Mammalia.
Before considering in detail the comparative development of the
nervous system, it will be convenient shortly to review the present
state of our knowledge on the general process of its evolution.
This process may be studied either embryologically, or by a comparison
of the various stages in its evolution preserved in living forms. Both
the methods have led to important results.
The embryological evidence shews 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,
while the central nervous system itself has arisen from the
concentration of such cells in special tracts. In the Chordata at any
rate the nerves arise as outgrowths of the central organ.
Another important fact shewn by embryology is that the central nervous
system, and percipient portions of the organs of special sense,
especially of optic organs, are often formed from the same part of the
primitive epidermis. Thus the retina of the Vertebrate eye is formed
from the two lateral lobes of the primitive fore-brain.
The same is true for the compound eyes of some Crustacea. The
supraoesophageal ganglia of these animals are formed in the embryo
from two thickened patches of the epiblast of the procephalic lobes.
These thickened patches become gradually detached from the surface,
remaining covered by a layer of epidermis. They then constitute the
supraoesophageal ganglia; but they form not only the ganglia, but also
the retinulæ of the eye--the parts in fact which correspond to the
rods and cones in our own retina. The accessory parts of these organs
of special sense, viz. the crystalline lens of the Vertebrate eye, and
the corneal lenses and crystalline cones of the Crustacean eye, are
independently formed from the epiblast after the separation of the
part which becomes the central nervous system.
In the Acraspedote Medusæ the rudimentary central nervous system has
the form of isolated rings, composed of sense-cells prolonged into
nervous fibres, surrounding the stalks of tentacle-like organs, at the
ends of which are placed the sense-organs.
This close connection between certain organs of special sense and
ganglia is probably to be explained by supposing that the two sets of
structures actually originated _pari passu_.
We may picture the process as being somewhat as follows:--
It is probable that in simple ancestral organisms the whole body was
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 the latter 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 is in fact very similar to the
rudimentary ganglia of the Acraspeda mentioned above. It is easy to
see by what steps it might become 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 of cells sensitive to light, and of ganglion-cells connected
with them; while at a later period optical structures, constituting
a lens capable of throwing an image of external objects upon it,
would be developed, 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 is often 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.
A series of forms of the Coelenterata and Platyelminthes affords us
examples of various stages in the differentiation of a central nervous
system[153].
[153] Our knowledge on this subject is especially due to the
brothers Hertwig (Nos. 320 and 321), Eimer (No. 318), Claus (No.
317), Schäfer (No. 326), and Hubrecht (No. 323).
In sea-anemones (Hertwigs, No. 321) 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 fine processes which penetrate into 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 characterised by sending a process into the superjacent
epithelium. Such cells are obviously intermediate between
neuro-epithelial cells and ganglion-cells; and it is probable that the
nerve-cells are, in fact, sense-cells which have travelled inwards and
lost their epithelial character.
In the Craspedote Medusæ (Hertwigs, No. 320) the differentiation of
the nervous system is carried somewhat further. There is here a
definite double ring, placed at the insertion of the velum, and
usually connected with sense-organs. The two parts of the ring belong
respectively to the epithelial layers on the upper and lower surfaces
of the velum, and are not separated from these layers; they are formed
of fine nerve-fibres and ganglion-cells. The epithelium above the
nerve rings contains sense-cells (fig. 237) with a stiff hair at their
free extremity, and a nervous prolongation at the opposite end, which
joins the nerve-fibres of the ring. Between such cells and true
ganglion-cells an intermediate type of cell has been found (fig. 237
B) which sends a process upwards amongst the epithelial cells, but
does not reach the surface. Such cells, as the Hertwigs have pointed
out, are clearly sense-cells partially transformed into
ganglion-cells.
A still higher type of nervous system has been met with amongst some
primitive Nemertines (Hubrecht, No. 323), consisting of a pair of
large cephalic ganglia, and two well-developed lateral ganglionic
cords placed close beneath the epidermis. These cords, instead of
giving off definite nerves, as in animals with a fully differentiated
nervous system, are connected with a continuous subdermal nervous
plexus.
[FIG. 236. NEURO-EPITHELIAL SENSE-CELLS OF AURELIA AURITA. (From
Lankester; after Schäfer.)]
The features of the embryology and the anatomy of the nervous system,
to which attention has just been called, point to the following
general conclusions as to the evolution of the nervous system.
(1) The nervous system of the higher Metazoa appears to have been
evolved in the course of a long series of generations 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.
(2) An early feature in the differentiation consisted in the growth of
a series of delicate processes of the inner ends of certain epithelial
cells, which became at the same time especially differentiated as
sense-cells (figs. 236 and 237).
[FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF
CARMARINA HASTATA. (After O. and R. Hertwig.)
A. Neuro-epithelial sense-cell. _c._ sense-hair.
B. Transitional cell between a neuro-epithelial cell and a
ganglion-cell.]
(3) These processes gave rise to a subepithelial nervous
plexus, in which ganglion-cells, formed from sense-cells which
travelled inwards and lost their epithelial character (fig. 237 B),
soon formed an important part.
(4) Local differentiations of the nervous network, which was
no doubt distributed over the whole body, took place partly in
the formation of organs of special sense, and partly in other
ways, and such differentiations gave rise to a central nervous
system. The central nervous system was at first continuous
with the epidermis, but became separated from it and travelled
inwards.
(5) 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.
The following points amongst others are still very obscure:--
(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.
It is probable, as stated in the above summary, 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.
[FIG. 238. MYOEPITHELIAL CELLS OF HYDRA. (From Gegenbaur; after
Kleinenberg.)
_m._ contractile fibres; processes of cells.]
The primitive relations between the nervous network and the muscular
system are matters of pure speculation. The primitive muscular cells
consist of epithelial cells with muscular processes (fig. 238), but
the branches of the nervous network have not been traced into
connection with the muscles in any Coelenterata except Ctenophora. In
the higher types a continuity between nerves and muscles in the form
of motorial end plates has been widely observed. Even in the case of
the Coelenterata it is quite clear from Romanes' experiments that
stimuli received by the nerves are capable of being transmitted to the
muscles, and that there must therefore be some connection between
nerves and muscles. How did this connection originate?
Epithelial cells with muscular processes (fig. 238) were discovered by
Kleinenberg (No. 324) in Hydra before epithelial cells with nervous
processes were known, and Kleinenberg pointed out that Hydra shewed
the possibility of nervous and muscular tissues existing without a
central nervous system, and suggested that the epithelial part of the
myoepithelial cells was a sense-organ, and that the connecting part
between this and the contractile processes was a rudimentary nerve. He
further supposed that in the subsequent evolution of these elements
the epithelial part of the cell became a ganglion-cell, while the part
connecting this with the muscular tail became prolonged so as to form
a true nerve. The discovery of neuro-epithelial cells existing side by
side with myoepithelial cells demonstrates that this theory must in
part be abandoned, and that some other explanation must be given of
the continuity between nerves and muscles. The hypothetical
explanation which most obviously suggests itself is that of fusion.
It seems quite possible that many of the epithelial cells of the
epidermis and walls of the alimentary tract 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. Such
cells would be very similar to Kleinenberg's neuro-muscular cells. By
a subsequent differentiation some of the cells forming this network
may have become specially contractile, the epithelial parts of the
cells ceasing to have a nervous function, and other cells may have
lost their contractility and become solely nervous. In this way we
should get neuro-epithelial cells and myoepithelial cells both
differentiated from the primitive network, and the connection between
the two would also be explained. This hypothesis fits in moreover very
well with the condition of the neuro-muscular system as we find it in
the Coelenterata.
BIBLIOGRAPHY.
_Origin of the Nervous System._
(316) F. M. Balfour. "Address to the Department of Anat. and Physiol.
of the British Association." 1880.
(317) C. Claus. "Studien üb. Polypen u. Quallen d. Adria. 1.
Acalephen, Discomedusen," _Denk. d. math.-naturwiss. Classe d. k.
Akad. Wiss. Wien_, Vol. XXXVIII. 1877.
(318) Th. Eimer. _Zoologische Studien a. Capri. 1. Ueber Beroë ovatus.
Ein Beitrag z. Anat. d. Rippenquallen._ Leipzig, 1873.
(319) V. Hensen. "Zur Entwicklung d. Nervensystems." _Virchow's
Archiv_, Vol. XXX. 1864.
(320) O. and R. Hertwig. _Das Nervensystem u. d. Sinnesorgane d.
Medusen._ Leipzig, 1878.
(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond.
Berücksichtigung d. Nervenmuskelsystem untersucht." _Jenaische Zeit._,
Vol. XIII. 1879.
(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." _Jenaische
Zeitschrift_, Vol. XIV. 1880.
(323) A. W. Hubrecht. "The Peripheral Nervous System in Palæo- and
Schizonemertini, one of the layers of the body-wall." _Quart. J. of
Micr. Science_, Vol. XX. 1880.
(324) N. Kleinenberg. _Hydra, eine
anatomisch-entwicklungsgeschichtliche Untersuchung._ Leipzig, 1872.
(325) A. Kowalevsky. "Embryologische Studien an Würmern u.
Arthropoden." _Mém. Acad. Pétersbourg_, Series VII., Vol. XVI. 1871.
(326) E. A. Schäfer. "Observations on the nervous system of Aurelia
aurita." _Phil. Trans._ 1878.
Nervous system of the Invertebrata. Our knowledge of the development
of the central nervous system is still very imperfect in the case of
many Invertebrate groups. In the Echinodermata and some of the
Chætopoda it is never detached from the epidermis, and in such cases
its origin is clear without embryological evidence.
In the majority of groups the central nervous system may be reduced to
the type of a pair of cephalic ganglia, continued posteriorly into two
cords provided with nerve-cells, which may coalesce ventrally or be
more or less widely separated, and be unsegmented or segmented.
Various additional visceral ganglia may be added, and in different
instances parts of the system may be much reduced, or peculiarly
modified. The nervous system of the Platyelminthes (when present), of
the Rotifera, Brachiopoda, Polyzoa (?), the Mollusca, the Chætopoda,
the Discophora, the Gephyrea, the Tracheata, and the Crustacea, the
various small Arthropodan phyla (Poecilopoda, Pycnognida, Tardigrada,
&c.), the Chætognatha (?), and the Myzostomea, probably belongs to
this type.
The nervous system of the Echinodermata cannot be reduced to this
form; nor in the present state of our knowledge can that of the
Nematelminthes or Enteropneusta.
It is only in the case of members of the former set of groups that any
adequate observations have yet been made on the development of the
nervous system, and even in the case of these groups observations
which have any claim to completeness are confined to certain members
of the Chætopoda, the Arthropoda and the Mollusca. An account of
imperfect observations on other forms, where such have been made, will
be found in the systematic part of this work.
Chætopoda. We are indebted to Kleinenberg (No. 329) for the most
detailed account which we have of the development of the central
nervous system in the Chætopoda.
[FIG. 239. SECTION THROUGH THE HEAD OF A YOUNG EMBRYO OF LUMBRICUS
TRAPEZOIDES. (After Kleinenberg.)
_c.g._ cephalic ganglion; _cc._ cephalic portion of the body cavity;
_x._ oesophagus.]
The supraoesophageal ganglion with the oesophageal commissure
developes independently of the ventral cord. It arises as an unpaired
thickening of the epiblast, close to the dorsal side of the oesophagus
at the front end of the head (fig. 239), which becomes separated from
the epiblast, and extends obliquely backwards and downwards in a
somewhat arched form; its lower extremities being somewhat swollen.
The inner portion of this curved rudiment becomes converted into
commissural nerve-fibres, while the cells of the outer and upper
portion assume the characters of ganglion-cells. The commissural
fibres are continued downwards to meet the ventral chord, but their
junction with the latter structure is not effected till late in
embryonic life.
The ventral cord is formed by the coalescence of a pair of linear
cords, the development of which takes place from before backwards, so
that when their anterior part is well developed their posterior part
is hardly differentiated. These cords arise, one on each side of a
ventral ciliated furrow, first as a single row of epiblast cells, and
subsequently as several rows (fig. 240, _Vg_). While still united to
the external epiblast, they extend themselves below the cells lining
the ventral furrow, and unite into a single nervous band, which
however exhibits its double origin by its bilobed section. Before the
two cords unite, the groove between them becomes somewhat deep, but
subsequently shallows out and disappears. The nervous band, before
separating from the epiblast, exhibits, in correspondence with the
mesoblastic segments, alternate swellings and constrictions. The
former become the ganglia, and the latter the connecting trunks.
[FIG. 240. SECTION THROUGH PART OF THE VENTRAL WALL OF THE TRUNK OF
AN EMBRYO OF LUMBRICUS TRAPEZOIDES. (After Kleinenberg.)
_m._ longitudinal muscles; _so._ somatic mesoblast; _sp._ splanchnic
mesoblast; _hy._ hypoblast; _Vg._ ventral nerve-cord; _vv._ ventral
vessel.]
As soon as the cord becomes free from the epiblast, it becomes
surrounded by a sheath, formed of somatic mesoblast. In each of the
ganglionic enlargements there next appears on the dorsal surface a
pair of areas of punctiform material, the substance of which soon
differentiates itself into nerve-fibres. These areas, by uniting from
side to side, give rise to the transverse commissures, and also by a
linear coalescence to the longitudinal commissures of the cord. The
cellular parts of the band surrounding them become converted into a
ganglionic covering of the cord.
In each ganglion the cells of this ganglionic investment penetrate as
a median septum into the cord. A fissure is next formed, dividing this
septum into two; it is subsequently continued for the whole length of
the cord.
Arthropoda. In the Tracheata and the Crustacea the development of the
ventral cord is in the main similar to that in the Chætopods, while
that of the supraoesophageal ganglia is as a rule somewhat more
complicated. No such clear evidence of an independent development of
these two parts, as in the case of the Chætopods, has as yet been
produced.
The most primitive type of nervous system amongst the Tracheata is
that of Peripatus, where it consists of large supraoesophageal
ganglia, continuous with a pair of widely separated but large ventral
cords united posteriorly above the anus. These cords have an
investment of ganglion-cells for their whole length, and are
imperfectly divided into ganglia corresponding in number with the
feet.
[FIG. 241. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS. The
embryo from which the section is taken was somewhat younger than
that of fig. 242.
_sp.m._ splanchnic mesoblast; _s.m._ somatic mesoblast; _mc._ median
section of body cavity; _lc._ lateral section of body cavity; _v.n._
ventral nerve cord; _me._ mesenteron.]
The ventral cords are formed as two separate epiblastic ridges (fig.
241, _v.n_), continued in front into a pair of thickenings of the
procephalic lobes, which are at first independent of each other, and
from which a large part of the supraoesophageal ganglia takes its
origin. After the latter have become separated from the epiblast an
invagination of the epiblast covering them grows into each lobe (fig.
242), and becoming constricted from the superficial epiblast, which
remains as the epidermis, forms a not unimportant part of the
permanent supraoesophageal ganglia.
In the Arachnida the mode of development of the nervous system is
essentially the same, and the reader will find a detailed account of
it for Spiders in Vol. II. pp. 447-451. The ventral cords are here
formed as independent and at first widely separated strands (fig. 243,
_vn_), which for a long time remain far apart; they are subsequently
divided into ganglia and become united by transverse commissures.
The supraoesophageal ganglia are formed as two independent thickenings
of the procephalic lobes (fig. 244), which eventually separate from
the superficial skin. There is formed however in each of them a
semicircular groove (fig. 244, _gr_) lined by the superficial
epiblast, which becomes detached from the skin, and is involuted to
form part of the ganglia.
[FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)
The figure shews the jaws (mandibles), and close to them epiblastic
involutions, which grow into the supraoesophageal ganglia. The
antennæ, oral cavity, and oral papillæ are also shewn.]
[FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA
LABYRINTHICA.
The ventral cords have begun to be formed as thickenings of the
epiblast, and the limbs are established.
_me.s._ mesoblastic somite; _vn._ ventral nerve-cord; _yk._ yolk.]
A similar mode of formation of both the ventral cords and the
supraoesophageal ganglia obtains in Insects (fig. 245). The ventral
cords are however much less widely separated than in Spiders, and
early unite in the median line. In the supraoesophageal ganglia the
invaginated epiblast has in Lepidoptera (Hatschek) the form of a pit
on the dorsal border of the antennæ.
Hatschek states that there takes place an invagination of a median
part of the skin between the two ventral cords, for the details of
which I must refer the reader to Vol. II. p. 410. He has made more or
less similar statements for the earthworm, but his observations in
both instances are open to serious doubt.
[FIG. 244. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF
AGELENA LABYRINTHICA.
_st._ stomadæum; _gr._ section through semicircular groove in
procephalic lobe; _ce.s._ cephalic section of body cavity.]
[FIG. 245. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYO OF
HYDROPHILUS. (After Kowalevsky.)
A. Transverse section through an embryo in the region of one of the
stigmata.
B. Transverse section through an older embryo.
_vn._ ventral nerve-cord; _am._ amnion and serous membrane; _me._
mesoblast; _me.s._ somatic mesoblast; _hy._ hypoblast (?); _yk._
yolk-cells (true hypoblast); _st._ stigma of trachea.]
Full details as to the development of the nervous system in the
Crustacea are still wanting; a fairly complete account of what is
known on the subject is given in Vol. II. pp. 521-2. It appears that
the ventral cord may either arise as an unpaired thickening of the
epiblast (Isopoda), marked however by a shallow median furrow, or from
two cords which eventually coalesce[154]. It is not certain how far
the supraoesophageal ganglia are usually in the first instance
continuous with the ventral cord. In Astacus, the early stages of
which have been elaborately investigated by Reichenbach (No. 331),
they are stated to be so; the supraoesophageal ganglia are moreover
described by this author as having a somewhat complicated origin. Five
elements enter into their composition. There is first formed a pair of
pits on the procephalic lobes, which become very deep during the
Nauplius stage, and are continuous with a pair of epiblastic ridges
which pass round the mouth, and join the ventral cords just described.
The walls of the pits are believed to form a part of the embryonic
ganglia which gives rise to the retina as well as to the optic
ganglia. The ridges form the remainder of the ganglia and the
oesophageal commissures; while the fifth element is supplied by a
median invagination in front of the mouth, which appears at a much
later date than the other parts.
[154] Reichenbach (No. 331) holds that the walls of the groove
between the two strands of the ventral cords become invaginated
and assist in the formation of the ventral cord.
In the Isopoda supraoesophageal ganglia are stated to arise as
thickenings of the procephalic lobes, which become eventually detached
from the epidermis.
The ventral cord is at first unsegmented, but soon becomes partially
divided by a series of constrictions into a number of ganglia,
corresponding with the segments. The development of the commissural
and ganglionic portions takes place much as in the Chætopoda.
The Gephyrea approach closely the types so far dealt with, but the
ventral cord in the Inermia is formed as an unpaired thickening of the
epiblast. In Echiurus, as has been shewn by Hatschek in an interesting
paper on the larva of this species, published since the appearance of
the first volume, there is a pair of ventral cords[155]. In
correspondence with a general segmentation of the body, which is
subsequently lost, these cords become segmented. The two cords unite
in the median line, and Hatschek, in accordance with his general view
on this subject, states that their junction is effected by means of a
median cord of invaginated epiblast. The segmentation of the cords
subsequently becomes lost. The supraoesophageal ganglia arise as an
unpaired median thickening of the procephalic lobe. No traces of
segmentation in the ventral cord have been observed by Spengel in
Bonellia, and the supraoesophageal ganglion is formed in this genus as
an unpaired band.
[155] "Ueber Entwicklungsgeschichte d. Echiurus." _Arbeit. a. d.
zool. Instit. Wien_, Vol. III. 1880.
In all the groups above considered the nervous system clearly presents
the same type of development with various modifications.
It is formed of two parts, viz. (1) the supraoesophageal ganglia, and
(2) the ventral cord.
In the simpler forms, Chætopoda and Gephyrea, the supraoesophageal
ganglia are usually stated to be formed as an unpaired thickening at
the apex of the præoral lobe, which in most cases becomes subsequently
bilobed.
In the Arthropoda the unpaired præoral lobe of the Chætopoda is
replaced by the so-called procephalic lobes, which are themselves
bilobed; and the supraoesophageal ganglia are formed of two
independent halves; further complications in development are also
generally found.
There is not as yet sufficient evidence to decide whether the
supraoesophageal ganglia were primitively developed continuously with,
or independently of, the ventral cords.
The ventral cord appears in the embryo as _two independent unsegmented
strands_, although in a few cases (some Crustacea and Gephyrea) these
cords, by an abbreviation in development, arise as an unpaired median
thickening of the epiblast.
The form of nervous system of the Chætopoda, Arthropoda, and Gephyrea
is clearly therefore to be derived, as was first pointed out by
Gegenbaur, from a more or less similar type to that now found in the
Nemertines; and as suggested in the chapter on larval forms (_vide_ p.
378) may perhaps be derived from the elongation of a circular ring, of
which the anterior end has become developed into the supraoesophageal
ganglia, the lateral parts into the two lateral strands, while the
posterior part persists in some forms in the junction of the ventral
cords above the anus (Enopla and Peripatus).
Mollusca. While study of the anatomy of the nervous system of the
Mollusca, especially of certain primitive genera (Chiton, Haliotis,
Fissurella, &c.) leaves little doubt that it is formed on the same
type as that of the groups just spoken of, the development, so far as
our imperfect knowledge enables us to make definite statements on the
subject, is somewhat abnormal[156].
[156] _Vide_ Vol. II., pp. 273, 274.
In the Gasteropoda and Pteropoda the supraoesophageal ganglia appear
most probably to be developed either as paired thickenings of the
epiblast of the velar area, or as invaginated pits of the velar area,
which become detached from the surface, and then become solid
(Hyaleacea and Limax). In either case the supraoesophageal ganglia
appear to be developed quite independently of the pedal ganglia. The
latter, as might be anticipated, are earlier in their development and
more constant than the various visceral ganglia; and, if the views
above expressed are correct, are homologous with the ventral cord of
the Chætopods and Arthropods. Their actual development is very
imperfectly known.
The most precise statements on the subject, viz. those of Bobretzky
and Fol, would lead us to suppose that they arise in the mesoblast,
but it seems more probable that they are formed as thickenings of the
sides of the foot.
In the Cephalopods all the ganglia are stated to be differentiated in
the mesoblast (Lankester, Bobretzky).
Hatschek[157] has recently given a detailed description of the
development of the supraoesophageal and pedal ganglia of Teredo. He
finds that the former ganglia arise as an unpaired thickening of the
epiblast in the centre of the velar area, and the latter as an
unpaired thickening of the epiblast of the ventral side of the body
between the mouth and the anus. The two ganglia would thus seem to be
disconnected with each other in their development.
[157] "Ueber Entwicklungsgeschichte von Teredo." _Arbeit. a. d.
zool. Instit. Wien_, Vol. III. 1880.
(327) F. M. Balfour. "Notes on the development of the Araneina."
_Quart. J. of Micr. Science_, Vol. XX. 1880.
(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren." _Jenaische
Zeitschrift_, Vol. XI. 1877.
(329) N. Kleinenberg. "The development of the Earthworm, Lumbricus
Trapezoides." _Quart. J. of Micr. Science_, Vol. XIX. 1879.
(330) A. Kowalevsky. "Embryologische Studien an Würmern u.
Arthropoden." _Mém. Acad. Pétersbourg_, Series VIII., Vol. XVI. 1871.
(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d.
Flusskrebses." _Zeit. f. wiss. Zool._, Vol. XXIX. 1877.
THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA[158].
[158] For the development of the central nervous system in
Amphioxus and the Tunicata the reader is referred to the chapters
dealing with those two groups.
The formation of the cerebrospinal axis of the Chordata from the
medullary plate has already been treated at length (pp. 301-304).
Before entering into the consideration of the morphological value of
the various parts of this cord, it will be convenient to describe the
more important features of its ontogeny. For this purpose the two
parts into which the nervous axis becomes at an early period divided,
viz. the spinal cord and the brain, may be dealt with separately.
The Spinal Cord, shortly after the closure of the medullary canal,
has, in all the true Vertebrata, the form of an oval tube; the walls
of which are of a fairly uniform thickness, and are composed of
several rows of elongated cells. This cord, as development proceeds,
usually becomes vertically prolonged in transverse section, and the
central canal which it contains also becomes vertically elongated. The
variations in shape of the spinal canal are very great at different
periods and in different parts of the body, and an attempt to
chronicle them would appear, in the present state of our knowledge, to
be quite valueless[159]. Fig. 117, in which the spinal cord of the
chick of the third day is shewn in transverse section, illustrates the
character of the cord at the stage just described. Up to this time the
walls of the spinal canal have exhibited an uniform structure. A
series of changes now however takes place, which results in the
differentiation (1) of the epithelium of the central canal, (2) of the
grey matter of the cord, and (3) of the external coating of white
matter.
[159] Löwe (No. 341) holds that at an early stage of development
three regions can always be distinguished in any section of the
central canal, viz. (1) a ventral narrow slit, (2) a median
enlargement, and (3) a dorsal slit. Such a form can no doubt
often be observed, but my own observations do not lead me to
attach any special importance to it.
The relative time at which each of these parts becomes developed is
not constant in the different forms.
[FIG. 246. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK.
_pcw._ dorsal white column; _lcw._ lateral white column; _acw._
ventral white column; _c._ dorsal tissue filling up the part where
the dorsal fissure will be formed; _pc._ dorsal grey cornu; _ac._
anterior grey cornu; _ep._ epithelial cells; _agc._ anterior
commissure; _pf._ dorsal part of spinal canal; _spc._ ventral part
of spinal canal; _af._ anterior fissure.]
The white matter is apparently the result of a differentiation of the
outermost parts of the superficial cells of the cord into longitudinal
nerve-fibres, which remain for a long period without a medullary
sheath. These fibres appear in transverse sections as small dots. The
white matter forms a transparent investment of the grey matter and
would seem to contain neither nuclei nor cells[160]. The white matter
may from the first form only two masses, one on each side, forming a
layer on the _ventral and lateral_ parts of the spinal cord but not
extending to the dorsal surface (Elasmobranchii, fig. 185, _W_); or it
may form four patches, viz. an anterior and a posterior white column
on each side, which lie on a level with the origin of the anterior and
posterior nerve-roots (the Fowl, Human embryo, etc.). In whichever of
these forms the white matter appears, it is always, at first, a layer
of extreme tenuity, which rapidly increases in thickness in the
subsequent stages, and extends so as gradually to cover the whole cord
(fig. 246).
[160] This holds true at first for Elasmobranchii, but at a later
stage there are present numerous nerve-cells in the white matter,
so that the distinction between the white and grey matter becomes
much less marked than in higher types; in this respect
Elasmobranchii present an approximation to Amphioxus.
The anterior white commissure is formed very shortly after the first
appearance of the white matter. The grey matter and the central
epithelium are formed by a differentiation of the main mass of the
spinal cord. The outer cells lose their epithelial-like arrangement,
and, becoming prolonged into fibres, give rise to the grey matter,
while the innermost cells retain their primitive arrangement, and
constitute the epithelium of the canal. The process of formation of
the grey matter would appear to proceed from without inwards, so that
some of the cells, which have, on the formation of the grey matter, an
epithelial-like arrangement, subsequently become converted into true
nerve-cells.
As has already been mentioned, the central epithelium of the nervous
system probably corresponds with the so-called epidermic layer of the
epiblast.
The grey matter soon becomes prolonged dorsally and ventrally into the
posterior and anterior horns. Its fibres may especially be traced in
two directions:--(1) round the anterior end of the spinal canal,
immediately outside its epithelium and so to the grey matter on the
opposite side, forming in this way an anterior grey commissure,
through which a decussation of the fibres from the opposite sides is
effected: (2) dorsalwards along the outside of the lateral walls of
the canal.
There is at this period no trace of the ventral or dorsal fissure, and
the shape of the central canal is not very different to what it was at
an earlier period. This condition of the spinal cord is especially
instructive, as it is very nearly that which is permanent in
Amphioxus.
The next event of importance is the formation of the ventral or
anterior fissure. This owes its origin to a downgrowth of the anterior
horns of the cord on each side of the middle line. The two downgrowths
enclose between them a somewhat linear space--the anterior
fissure--which increases in depth in the succeeding stages (fig. 246,
_af_).
The dorsal or posterior fissure is formed at a later period than the
anterior, and accompanies the atrophy of the dorsal section of the
embryonically large canal of the spinal cord.
The exact mode of its formation appears to me to be still involved in
some obscurity.
In the _Elements of Embryology_ the development of the posterior
fissure was described in the following way:
"On the seventh day the most important event is the formation of the
_posterior fissure_.
"This is brought about by the absorption of the roof of the
posterior of the two parts into which the neural canal has become
divided.
"Between the posterior horns of the cord, the epithelium forming the
roof of the, so to speak, posterior canal is along the middle line
covered neither by grey nor by white matter, and on the seventh day
is partially absorbed, thus transforming the canal into a
wedge-shaped fissure, whose mouth however is seen in section to be
partially closed by a triangular clump of elongated cells (fig. 246,
_c_). Below this mass of cells the fissure is open. It is separated
from the 'true spinal canal' by a very narrow space along which the
side walls have coalesced. In the lumbar and sacral regions the two
still communicate.
"We thus find, as was first pointed out by Lockhart Clarke, that the
anterior and posterior fissures of the spinal cord are,
morphologically speaking, entirely different. The anterior fissure
is merely the space left between two lateral downward growths of the
cord, while the posterior fissure is part of the original neural
canal separated from the rest of the cavity (which goes to form the
true spinal canal) by a median coalescence of the side walls."
I confess that I have some doubts as to the complete accuracy of the
above statement.
Kölliker gives a full account of the gradual atrophy of the central
canal; but I do not fully understand his statements with reference to
the formation of the posterior fissure, which in fact appears to be
only incidentally mentioned. It would seem from his account that a
shallow and somewhat wide dorsal fissure is formed to start with, in
the human embryo, by two projections of the posterior white horns. On
the atrophy of the central canal this furrow becomes narrowed, but
Kölliker does not definitely state how it becomes deepened so as to
give rise to the permanent dorsal fissure.
It seems to me probable, though further investigations on the point
are still required, that the dorsal fissure is a direct result of the
atrophy of the dorsal part of the central canal of the spinal cord.
The walls of the canal coalesce dorsally, and the coalescence
gradually extends ventralwards, so as finally to reduce the central
canal to a minute tube, formed of the ventral part of the original
canal. The epithelial wall formed by the coalesced walls on the dorsal
side of the canal is gradually absorbed.
The epithelium of the central canal, at the period when its atrophy
commences, is not covered dorsally either by grey or white matter, so
that, with the gradual reduction of the dorsal part of the canal, and
the absorption of the epithelial wall formed by the fusion of its two
sides, a fissure between the two halves of the spinal cord becomes
formed. This fissure is the posterior or dorsal fissure. In the
process of its formation the white matter of the dorsal horns becomes
prolonged so as to line its walls; and shortly after its formation the
dorsal grey commissure makes its appearance, which is not improbably
derived from part of the epithelium of the original central canal.
_Development of the Brain._
The brain is formed from the anterior portion of the medullary plate.
When the medullary plate first becomes differentiated it is not
possible to distinguish between the region of the brain and that of
the spinal cord. The brain region is however usually very early
indicated by a widening of the medullary plate, but does not become
sharply marked off from the region of the spinal cord. In many
Ichthyopsida (Elasmobranchii (fig. 28, C) and Amphibia (fig. 77, A))
the anterior dilatation gives to the medullary plate, before its sides
meet to form a canal, a spatula-like form; which is either not present
or less marked in Reptilia, Aves and Mammalia.
The length of the brain as compared to the spinal cord is always very
great in the embryo, and in the earliest developmental periods the
disproportion in the size of the brain is specially marked, owing to
the full number of the somites of the trunk not having been formed. In
Elasmobranchii the brain is about one-third of the whole length of the
embryo at the stage immediately following the closure of the medullary
canal.
The first differentiation of the brain into distinct parts is a very
early occurrence, and may take place before (Mammalia) or during the
closure of the medullary folds. The brain first becomes divided into
two successive lobes or vesicles by a single transverse constriction,
and subsequently the posterior of these again becomes divided into
two, so that three lobes are formed--known as the fore- the mid- and
the hind-brain; of these the hind-brain is usually the longest. In
some instances a bilobed stage can hardly be recognised. This
primitive division of the brain is shewn in many of the figures
already given. The reader may perhaps best refer to fig. 108. On the
closure of the medullary groove the lumen of the medullary canal is
continued uninterruptedly through the brain, but dilates considerably
in each of the cerebral vesicles.
The anterior lobe of the brain becomes converted into the cerebral
hemispheres, the thalamencephalon, the primary optic vesicles, and the
parts connected with them. The middle lobe becomes the optic lobes
(corpora bigemina or corpora quadrigemina in Mammalia) and the crura
cerebri; while the posterior lobe becomes converted into the
cerebellum and medulla oblongata.
Before describing in detail the changes by which the primary vesicles
of the brain become converted into the above parts, it will be
convenient to say a few words about the general development of the
brain.
The most striking peculiarity with reference to the general
development of the brain is a curvature which appears in its axis,
known as the cranial flexure. The flexure takes place through the
mid-brain, and causes the fore-brain to be gradually bent downwards so
that the axis of its floor forms, first, a right angle with that of
the hinder part of the brain, and subsequently, as a rule, an acute
angle.
[FIG. 247. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.
_cer._ commencement of the cerebral hemisphere; _pn._ pineal gland;
_In._ infundibulum; _pt._ ingrowth from mouth to form the pituitary
body; _mb._ mid-brain; _cb._ cerebellum; _ch._ notochord; _al._
alimentary tract; _Iaa._ artery of mandibular arch.]
During these changes the brain, in most Amniota at any rate, becomes
in the first instance retort-shaped, the cerebral vesicle forming the
swollen part of the retort, but subsequently the retort-shape is lost
owing to the great development of the vesicle of the mid-brain, which
forms the termination of the long axis of the embryo. Figs. 29, 76,
and 118, are representative figures of embryos of various vertebrate
forms at a period when the mid-brain forms the termination of the long
axis of the body.
It is generally stated that the cranial flexure is at its maximum at
the stage represented in these figures, and there can be no doubt that
viewed from the exterior the cranial flexure ceases to be so marked a
feature, and finally disappears as the embryo gradually grows older;
but though the mid-brain ceases to form the termination of the long
axis of the embryo, the flexure of the brain becomes in many forms
absolutely more marked; while in other forms, though stated to
diminish, it does not entirely vanish.
[FIG. 248. LONGITUDINAL SECTION THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF DEVELOPMENT.
_cer._ cerebral hemisphere; _pn._ pineal gland; _op.th._ optic
thalamus, connected with its fellow by a commissure (the middle
commissure). In front of it is seen a fold of the roof of the
fore-brain, which is connected with the choroid plexus of the third
ventricle; _op._ optic chiasma; _pt._ pituitary body; _in._
infundibulum; _cb._ cerebellum; _au.v._ passage leading from the
auditory vesicle to the exterior; _mel._ medulla oblongata; _c.in._
internal carotid artery.]
The general nature of the changes which take place will perhaps best
be understood by a comparison of figs. 247 and 248 representing
longitudinal sections at two stages through the brain of an embryo
Elasmobranch. The actual cranial flexure, _i.e._ flexure of the floor
of the brain, is obviously greater in the older of the two brains,
though viewed from the exterior the axis of this brain appears to be
quite straight. In the younger stage, fig. 247, the mid-brain (_mb_)
forms the end of the long axis of the body, while in the older one the
cerebral hemispheres (_cer_) have grown very greatly, especially
forwards and dorsalwards. They have thus come to lie in front of the
mid-brain, and to form the end of the long axis of the body, and have
at the same time compressed the originally large thalamencephalon
against the mid-brain. The same general features may be seen in fig.
250 representing a longitudinal section of the brain of an embryo
fowl, and fig. 255 representing a longitudinal section of the brain of
a Mammal.
The infundibulum or perhaps rather the point of origin of the optic
nerves is to be regarded as the anterior termination of the axis of
the base of the brain.
The cranial flexure is least marked in Cyclostomata (fig. 253),
Teleostei, Ganoidei, and Amphibia, while it is very pronounced in
Elasmobranchii, Reptilia, Aves, and Mammalia. In Teleostei, and still
more in Cyclostomata, it permanently remains slight, owing to the
small development of the cerebral hemispheres.
In addition to the cranial flexures, two other flexures make their
appearance in the base of the brain. A posterior at the junction of
the brain and spinal cord, and an anterior at the boundary between the
cerebellum and medulla oblongata, just at the point where the pons
Varolii is formed in Mammalia. The anterior of these is the most
marked and constant; it is shewn in fig. 250. It arises considerably
later than the main cranial flexure, and since it is turned the
opposite way it assists to a considerable extent in causing the
apparent straightening of the cranial axis.
Histogenetic changes[161]. The walls of the brain are at first very
thin and, like those of the spinal cord, are formed of a number of
ranges of spindle-shaped cells. The processes of each of these cells
are stated to be continued through the whole thickness of the wall. In
the floor of the hind- and mid-brain a superficial layer of delicate
nerve-fibres is formed at an early period. This layer appears in the
first instance on the floor and sides of the hind-brain, and very
slightly, if at all, later on the floor and the sides of the
mid-brain. The cells internal to the nerve-fibres become
differentiated into an innermost epithelial layer lining the cavities
of the ventricles, and an outer layer of grey matter.
[161] It is not within the scope of this work to give an account
of the histogenesis of the brain; in the statement in the text
only a few points, of some morphological importance, are touched
on.
The similarity of the primitive arrangement and histological character
of the parts of the brain behind the cerebral hemispheres to that of
the spinal cord is very conclusively shewn by the examination of any
good series of sections. In both brain and spinal cord the white
matter forms a cap on the ventral and lateral parts considerably
before it extends to the dorsal surface. In the medulla the white
matter does not eventually extend to the roof owing to the peculiar
degeneration which that part undergoes.
In the case of the fore-brain the earliest histological changes,
except possibly in Mammals, take place on the same general plan as
those of the remainder of the central nervous system[162]; but though
the general plan is the same, yet the early histological distinction
between the fore-brain, and the mid- and hind-brain is more marked
than the distinction between the latter and the spinal cord.
[162] I have worked out these changes in Elasmobranchii, Amphibia
(Salamandra) and Aves.
On the floor and sides of the thalamencephalon, and apparently the
whole of the hemispheres of the lower types, there is formed, somewhat
later than in the remainder of the brain, a very delicate layer of
white matter. The inner part of the wall, which still remains
comparatively thin, is not at first clearly divided into an epithelial
and nervous layer. This distinction soon however becomes more or less
apparent, though it is not so marked as in most other parts of the
brain; and it appears that in the subsequent growth the greater part
of the original epithelial layer becomes converted into nervous
tissue.
In Mammals the same plan of differentiation would seem to be followed,
though somewhat less obviously than in the lower types. The walls of
the hemispheres become first divided (Kölliker) into a superficial
thinner layer of rounded elements, and a deeper and thicker epithelial
layer, and between these the fibres of the crura cerebri soon
interpose themselves. At a slightly later period a thin superficial
layer of white matter, homologous with that of the remainder of the
brain, becomes established.
The inner layer, together with the fibres from the crura cerebri,
gives rise to the major part of the white matter of the hemispheres
and to the epithelium lining the lateral ventricles.
The outer layer of rounded cells becomes divided into (1) a
superficial part with comparatively few cells, which, together with
its coating of white matter, forms the cortical part of the grey
matter, and (2) a deeper layer with numerous cells which forms the
main mass of the grey matter of the hemispheres.
The development of the several parts of the brain will now be
described.
The hind-brain. The hind-brain is at first an elongated, funnel-shaped
tube, the walls of which are of a nearly uniform thickness, though the
roof and floor are somewhat thinner than the sides. It forms a direct
continuation of the spinal cord, into which it passes without any
sharp line of demarcation. The ventricle it contains is known as the
fourth ventricle.
The sides become in the chick marked by a series of transverse
constrictions, dividing it into lobes, which are somewhat indefinite
in number. The first of these remains permanent, and its roof gives
rise to the cerebellum. It is uncertain whether the other
constrictions have any morphological significance. More or less
similar constrictions are present in Teleostei. In Elasmobranchii the
medulla presents on its inner face at a late period a series of lobes
corresponding with the roots of the vagus and glossopharyngeal nerves,
and it is possible that the earlier constrictions may potentially
correspond to so many nerve-roots.
[FIG. 249. SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF
THE THIRD DAY OF INCUBATION.]
_IV._ Fourth ventricle. The section shews the very thin roof and
thicker sides of the ventricle. _Ch._ Notochord; _CV._ Anterior
cardinal vein; _CC._ Involuted auditory vesicle; _CC_ points to the
end which will form the cochlear canal; _RL_. Recessus labyrinthi
(remains of passage connecting the vesicle with the exterior); _hy._
Hypoblast lining the alimentary canal; _AO._, _AOA._ Aorta, and
aortic arch.]
Throughout the Vertebrata an anterior lobe of the hind-brain becomes
very early marked off, so that the primitive hind-brain becomes
divided into two regions which may be conveniently spoken of as the
cerebellum (figs. 247 and 248, _cb_) and medulla oblongata. The floor
of these regions is quite continuous and is also prolonged without any
break into the floor of the mid-brain.
The posterior section of the hind-brain, which forms the medulla,
undergoes changes of a somewhat complicated character. In the first
place its roof becomes in front very much extended and thinned out. At
the raphe, where the two lateral halves of the brain originally
united, a separation, as it were, takes place, and the two sides of
the brain become pushed apart, remaining united by only a very thin
layer of nervous matter, consisting of a single row of flattened cells
(fig. 249). As a result of this peculiar growth in the brain, the
roots of the nerves of the two sides, which were originally in contact
at the dorsal summit of the brain, become carried away from one
another, and appear to arise at the sides of the brain.
The thin roof of the fourth ventricle is triangular, or, in Mammalia,
somewhat rhomboidal in shape. The apex of the triangle is directed
backwards.
At a later period the blood-vessels of the pia mater form a rich
plexus over the anterior part of the thin roof of the medulla, which
becomes at the same time somewhat folded. The whole structure is known
as the tela vasculosa, or choroid plexus of the fourth ventricle (fig.
250, _chd_ 4). The floor of the whole hind-brain becomes thickened,
and there very soon appears on its outer surface a layer of
non-medullated nerve-fibres, similar to those which first appear on
the spinal cord. They are continuous with a similar layer of fibres on
the floor of the mid-brain, where they constitute the crura cerebri.
On the ventral floor of the medulla is a shallow continuation of the
anterior fissure of the spinal cord.
In Elasmobranchii and many Teleostei the restiform tracts are well
developed, and are anteriorly continued into the cerebellum, of which
they form the peduncles. Near their junction with the cerebellum they
form prominent bodies, which are regarded by Miklucho-Maclay as
representing the true cerebellum of Elasmobranchii.
In Elasmobranchii a dorsal pair of ridges projects into the cavity of
the fourth ventricle, corresponding apparently with the fasciculi
teretes of the Mammalia.
In Mammalia there develop, subsequently to the longitudinal fibres
already spoken of, first the olivary bodies of the ventral side of the
medulla, and at a still later period the pyramids. The fasciculi
teretes in the cavity of the fourth ventricle are developed shortly
before the pyramids.
When the hind-brain becomes divided into two regions the roof of the
anterior part does not become thinned out like that of the posterior,
but on the contrary, becomes somewhat thickened and forms a band-like
structure roofing over the anterior part of the fourth ventricle (fig.
247 and fig. 253, _cb_).
This is a rudiment of the cerebellum, and in all Craniate Vertebrates
it at first presents this simple structure and insignificant size. In
Cyclostomata, Amphibia and many Reptilia this condition is permanent.
In Elasmobranchii, on the other hand, the cerebellum assumes in the
course of development a greater and greater prominence (fig. 248,
_cb_), and eventually overlaps both the optic lobes in front and the
medulla behind. In the later embryonic stages it exhibits in
surface-views the appearance of a median constriction, and the portion
of the ventricle contained in it is prolonged into two lateral
outgrowths.
Miklucho-Maclay, from his observations on the brains of adult
Elasmobranchii, was led to regard what is here called the cerebellum
as identical with the mid-brain, and the true mid-brain as part of the
thalamencephalon. Miklucho-Maclay was no doubt misled by the large
size of the cerebellum, but, as we have seen, this body does not begin
to be conspicuous till late in embryonic life.
The mid-brain and thalamencephalon (according to the ordinary
interpretations) have in the embryo of Elasmobranchs exactly the same
relations as in the embryos of other Vertebrates; so that the
embryological evidence appears to me to be conclusive against
Miklucho-Maclay's view.
In Birds the cerebellum attains a very considerable development (fig.
250, _cbl_), consisting of a folded central lobe with an arbor vitæ,
into which the fourth ventricle is prolonged. There are two small
lateral lobes, apparently equivalent to the flocculi. Anteriorly the
cerebellum is connected with the roof of the mid-brain by a delicate
membrane, the velum medullæ anterius, or valve of Vieussens (fig. 250,
_vma_). The pons Varolii of Mammalia is represented by a small number
of transverse fibres on the floor of the hind-brain immediately below
the cerebellum.
In Mammalia the cerebellum attains a still greater development. The
median lobe or vermiform process is first developed. In the higher
Mammalia the lateral parts forming the hemispheres of the cerebellum
become formed as swellings at the sides at a considerably later
period, and are hardly developed in the Monotremata and Marsupialia.
[FIG. 250. LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF TEN
DAYS. (After Mihalkovics.)
_hms._ cerebral hemispheres; _alf._ olfactory lobe; _alf_{{1}}.
olfactory nerve; _ggt._ corpus striatum; _oma._ anterior commissure;
_chd{{3}}_. choroid plexus of the third ventricle; _pin._ pineal
gland; _cmp._ posterior commissure; _trm._ lamina terminalis; _chm._
optic chiasma; _inf._ infundibulum; _hph._ pituitary body; _bgm._
commissure of Sylvius (roof of iter a tertio ad quartum
ventriculum); _vma._ velum medullæ anterius (valve of Vieussens);
_cbl._ cerebellum; _chd{{4}}_. choroid plexus of the fourth
ventricle; _obl{{4}}_. roof of fourth ventricle; _obl._ medulla
oblongata; _pns._ commissural part of medulla; _inv._ sheath of
brain; _bls._ basilar artery; _crts._ internal carotid.]
The cerebellum is connected with the roof of the mid-brain in front
and with the choroid plexus of the fourth ventricle behind by delicate
membranous structures, known as the velum medullæ anterius (valve of
Vieussens) and the velum medullæ posterius.
The pons Varolii is formed on the ventral side of the floor of the
cerebellar region as a bundle of transverse fibres at about the same
time as the olivary bodies.
The mid-brain. The changes undergone by the mid-brain are simpler than
those of any other part of the brain. We have already seen that the
mid-brain, on the appearance of the cranial flexure, forms _an
unpaired vesicle_ with a vaulted roof and curved floor, at the front
end of the long axis of the body (fig. 118, _MB_). It is at this
period in most Vertebrates relatively much larger than in the adult;
and it is only in the Teleostei that it more or less retains in the
adult its embryonic proportions.
The cavity of the mid-brain, greatly reduced in size in the higher
forms, is known as the iter a tertio ad quartum ventriculum, or
aqueductus Sylvii.
The roof of the mid-brain is sharply constricted off from the
divisions of the brain in front of and behind it, but these
constrictions do not extend to the floor.
In some Vertebrates the region of the mid-brain is stated to undergo
hardly any further development. In the Axolotl it remains according to
Stieda[163] as a simple tube with nearly uniformly thick walls. In the
majority of forms it undergoes, however, a more complicated
development.
[163] "Ueb. d. Bau d. centralen Nervensystems d. Axolotl." _Zeit.
f. wiss. Zool._, Vol. xxv. 1875.
In Elasmobranchs the sides become thickened to form the optic lobes,
which are soon separated by a median longitudinal groove. The floor
becomes thickened to form the crura cerebri. The primitive simple
median cavity becomes imperfectly divided into a median portion below,
and two lateral diverticula in the optic lobes.
In Teleostei the changes, resulting in the formation of (1) a pair of
longitudinal ridges projecting from the roof into the cavity of the
iter, constituting the fornix of Gottsche, and (2) of the two
swellings on the floor, forming the tori semicirculares, are more
complicated, but have not been satisfactorily worked out. In
Bombinator and the Anura generally the changes are of the same nature
as those in Elasmobranchii, except that the prolongations of the
ventricle into the optic lobes are still further constricted off from
the median portion, which forms the true iter.
In Reptilia and Aves the development of the mid-brain takes place on
the same type as in Elasmobranchii and the Anura. In Birds the optic
lobes are pushed very much aside, and the roof of the iter is greatly
thinned out. In Mammalia the sides of the mid-brain give rise to two
pairs of prominences--the corpora quadrigemina--instead of the two
optic lobes of other Vertebrata. The prominences, which do not contain
prolongations of the iter, become first visible on the appearance of
an oblique transverse furrow, while the anterior pair alone are
separated by a longitudinal furrow. In the later stages of development
the longitudinal furrow is continued so as to bisect the posterior
pair.
The floor, which is bounded posteriorly by the pons Varolii, becomes
the crura cerebri. The corpora geniculata interna also belong to this
division of the brain.
Fore-brain. In its earliest condition the fore-brain forms a single
vesicle without a trace of separate divisions, but very early it buds
off the optic vesicles, whose history is described with that of the
eye.
[FIG. 251. SECTION THROUGH THE FRONT PART OF THE HEAD OF A
LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION.
_al._ alimentary tract; _fb._ thalamencephalon; _l._ lens of eye;
_op.v._ optic vesicle. The mesoblast is not represented.]
[FIG. 252. LONGITUDINAL SECTION THROUGH THE BRAIN OF A YOUNG
PRISTIURUS EMBRYO.
_cer._ commencement of cerebral hemisphere; _pn._ pineal gland;
_In._ infundibulum; _pt._ ingrowth of mouth to form the pituitary
body; _mb._ mid-brain; _cb._ cerebellum; _ch._ notochord; _al._
alimentary tract; _Iaa._ artery of mandibular arch.]
The optic vesicles become gradually constricted off from the
fore-brain in a direction obliquely backwards and downwards. They
remain, however, attached to it at the anterior extremity of the base
of the fore-brain (fig. 251, _op.v._). While the above changes are
taking place in the optic vesicles the anterior part of the fore-brain
becomes prolonged, and at the same time somewhat dilated. At first
there is no sharp boundary between the primitive fore-brain and its
anterior prolongation, but there shortly appears a constriction which
passes from above obliquely forwards and downwards. This constriction
is shallow at first, but soon becomes much deeper, leaving however the
cavities of the two divisions of the fore-brain united ventrally by a
somewhat wide canal (fig. 252).
Of these two divisions the posterior becomes the thalamencephalon,
while the anterior and larger division (_cer_) forms the rudiment of
the cerebral hemispheres and olfactory lobes. For a considerable
period this rudiment remains perfectly simple, and exhibits no signs,
either externally or internally, of a longitudinal constriction
dividing it into two lobes.
From the above description it may be concluded that the rudiment of
the cerebral hemispheres is contained in the original fore-brain. In
spite however of their great importance in all the Craniata, it is
probable that the hemispheres were either not present as distinct
structures, or only imperfectly separated from the thalamencephalon,
in the primitive vertebrate stock.
The thalamencephalon. The thalamencephalon varies so slightly in
structure throughout the Vertebrate series that a general description
will suffice for all the types.
It forms at first a simple vesicle, the walls of which are of a nearly
uniform thickness and formed of the usual spindle-shaped cells.
[FIG. 253. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A LARVA
OF PETROMYZON.
The larva had been hatched three days, and was 4.8 mm. in length.
The optic and auditory vesicles are supposed to be seen through the
tissues.
_c.h._ cerebral hemisphere; _th._ optic thalamus; _in._
infundibulum; _pn._ pineal gland; _mb._ mid-brain; _cb._ cerebellum;
_md._ medulla oblongata; _au.v._ auditory vesicle; _op._ optic
vesicle; _ol._ olfactory pit; _m._ mouth; _br.c._ branchial pouches;
_th._ thyroid involution; _v.ao._ ventral aorta; _ht._ ventricle of
heart; _ch._ notochord.]
The cavity it contains is known as the third ventricle. Anteriorly it
opens widely into the cerebral rudiment, and posteriorly into the
ventricle of the mid-brain. The opening into the cerebral rudiment
becomes the foramen of Munro.
For convenience of description I shall divide it into three regions,
viz. (1) the floor, (2) the sides, and (3) the roof.
The floor becomes divided into two parts, an anterior part, giving
origin to the optic nerves, in which is formed the optic chiasma; and
a posterior part, which becomes produced into an at first
inconspicuous prominence--the rudiment of the infundibulum (fig. 252,
_In_). This comes in contact with an involution from the mouth, which
gives rise to the pituitary body (fig. 252, _pt_), the development of
which will be dealt with separately.
In the later stages of development the infundibulum becomes gradually
prolonged, and forms an elongated diverticulum of the third ventricle,
the apex of which is in contact with the pituitary body (figs. 252,
254, _in_, and figs. 250 and 255, _inf_).
Along the sides of the infundibulum run the commissural fibres
connecting the floor of the mid-brain with the cerebrum.
In its later stages the infundibular region presents considerable
variations in the different vertebrate types. In Fishes it generally
remains very large, and permanently forms a marked diverticulum of the
floor of the thalamencephalon. In Elasmobranchii the distal end
becomes divided into three lobes--a median and two lateral. The
lateral lobes appear to become the sacci vasculosi of the adult.
In Teleostei peculiar bodies known as the lobi inferiores (hypoaria)
make their appearance at the sides of the infundibulum. They appear to
correspond in position with the tuber cinereum of Mammalia[164]. In
Birds, Reptiles, and Amphibia the lower part of the embryonic
infundibulum becomes atrophied and reduced to a mere finger-like
process--the processus infundibuli.
[164] For the relations of these bodies, _vide_ L. Stieda, "Stud.
üb. d. centrale Nervensystem d. Knochenfische." _Zeit. f. wiss.
Zool._ Vol. XVIII. 1868.
In Mammalia the posterior part of the primitive infundibulum becomes
the corpus albicans, which is double in Man and the higher Apes; the
ventral part of the posterior wall forms the tuber cinereum.
Laterally, at the junction of the optic thalami and infundibulum,
there are placed the fibres of the crura cerebri, which are probably
derived from the walls of the infundibulum. A special process grows
out from the base of the infundibulum, which undergoes peculiar
changes, and becomes intimately united with the pituitary body; in
which connection it will be more fully described.
[FIG. 254. LONGITUDINAL SECTION THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF DEVELOPMENT.
_cer._ cerebral hemisphere; _pn._ pineal gland; _op. th._ optic
thalamus, connected with its fellow by a commissure (the middle
commissure). In front of it is seen a fold of the roof of the
fore-brain, which is the choroid plexus of the third ventricle;
_op._ optic chiasma; _pt._ pituitary body; _in._ infundibulum; _cb._
cerebellum; _au.v._ passage leading from the auditory vesicle to the
exterior; _mel._ medulla oblongata; _c.in._ internal carotid
artery.]
The sides of the thalamencephalon become very early thickened to form
the optic thalami, which constitute the most important section of the
thalamencephalon. They are separated, in Mammalia at all events, on
their inner aspect from the infundibular region by a somewhat S-shaped
groove, known as the sulcus of Munro, which ends in the foramen of
Munro. They also become in Mammalia secondarily united by a transverse
commissure, the grey or middle commissure, which passes across the
cavity of the third ventricle. This commissure is probably homologous
with, and derived from, a commissural band in the roof of the
thalamencephalon, placed immediately in front of the pineal gland
which is well developed in Elasmobranchii (fig. 254).
The roof undergoes more complicated changes. It becomes divided, on
the appearance of the pineal gland as a small papilliform outgrowth
(the development of which is dealt with separately), into two
regions--a longer anterior in front of the pineal gland and a shorter
posterior. The anterior region becomes at an early period excessively
thin, and at a later period, when the roof of the thalamencephalon is
shortened by the approach of the cerebral hemispheres to the
mid-brain, it becomes (_vide_ figs. 250 and 255, _chd_ 3, and 254)
considerably folded, while at the same time a vascular plexus is
formed in the pia mater above it. On the accomplishment of these
changes it is known as the tela choroidea of the third ventricle.
In the roof of the third ventricle behind the pineal gland there
appear in Elasmobranchii, the Sauropsida and Mammalia transverse
commissural fibres, forming a structure known as the posterior
commissure, which connects together the two optic thalami.
The most remarkable organ in the roof of the thalamencephalon is the
pineal gland, which is developed in most Vertebrates as a simple
papilliform outgrowth of the roof, and is at first composed of cells
similar to those of the other parts of the central nervous system
(figs. 250, 252, 254 and 255, _pn_ or _pin_). In the lower Vertebrata
it is directed forwards, but in Mammalia, and to some extent in Aves,
it is directed backwards.
In Amphibia it is described by Götte (No. 296) as being a product of
the point where the roof of the brain remains latest attached to the
external skin.
The figure which Götte gives to prove this does not appear to me fully
to bear out his conclusion; which if true is very important. Although
I directed my attention specially to this point, I could find no
indication in Elasmobranchii of a process similar to that described by
Götte, and his observations have not as yet been confirmed for other
Vertebrates. Götte compares the pineal gland to the long-persisting
pore which leads into the cavity of the brain in the embryo of
Amphioxus, and we might add the Ascidians, and, should his facts be
confirmed, the conclusion he draws from them would appear to be well
founded.
The later stages in the development of the pineal gland in different
Vertebrates have not in all cases been fully worked out[165].
[165] For a full account of this subject _vide_ Ehlers (No. 337).
[FIG. 255. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART
OF THE BRAIN OF AN EMBRYO RABBIT OF FOUR CENTIMETRES. (After
Mihalkovics.)
The section passes through the median line so that the cerebral
hemispheres are not cut; their position is however indicated in
outline.
_spt._ septum lucidum formed by the coalescence of the inner walls
of part of the cerebral hemispheres; _cna._ anterior commissure;
_frx._ vertical pillars of the fornix; _cal._ genu of corpus
callosum; _trm._ lamina terminalis; _hms._ cerebral hemispheres;
_olf._ olfactory lobes; _acl._ artery of corpus callosum; _fmr._
position of foramen of Munro; _chd{{3}}._ choroid plexus of third
ventricle; _pin._ pineal gland; _cmp._ posterior commissure; _bgm._
lamina uniting the lobes of the mid-brain; _chm._ optic chiasma;
_hph._ pituitary body; _inf._ infundibulum; _pns_. pons Varolii;
_pde._ cerebral peduncles; _agd._ iter.]
In Elasmobranchii the pineal gland becomes in time very long, and
extends far forwards over the roof of the cerebral hemispheres (fig.
254 _pn_). Its distal extremity dilates somewhat, and in the adult the
whole organ forms (Ehlers, No. 337) an elongated tube, enlarged at its
free extremity, and opening at its base into the brain. The enlarged
extremity may either be lodged in a cavity in the cartilage of the
cranium (Acanthias), or be placed outside the cranium (Raja).
In Petromyzon its form is very different. It arises (fig. 253 _pn_) as
a sack-like diverticulum of the thalamencephalon extending at first
both backwards and forwards. In the Ammocoete the walls of this sack
are deeply infolded.
The embryonic form of the pineal gland in Amphibia is very much like
that which remains permanent in Elasmobranchii; the stalk connecting
the enlarged terminal portion with the brain soon however becomes
solid and very thin except at its proximal extremity. The enlarged
portion also becomes solid, and is placed in the adult externally to
the skull, where it forms a mass originally described by Stieda as the
cerebral gland.
In Birds the primitive outgrowth to form the pineal gland becomes,
according to Mihalkovics, deeply indented by vascular connective
tissue ingrowths, so that it assumes a dendritic structure (fig. 250
_pin_).
The proximal extremity attached to the roof of the thalamencephalon
forms a special section, known as the infra-pineal process. The
central lumen of the free part of the gland finally atrophies, but the
branches still remain hollow. The infra-pineal process becomes reduced
to a narrow stalk, connecting the branched portion of the body with
the brain. The branched terminal portion and the stalk obviously
correspond with the vesicle and distal part of the stalk of the types
already described. In Mammalia the development of the pineal gland is,
according to Mihalkovics, generally similar to that of Birds. The
original outgrowth becomes branched, but the follicles or lobes to
which the branching gives rise eventually become solid (fig. 255
_pin_). An infra-pineal process is developed comparatively late, and
is not sharply separated from the roof of the brain.