DEVELOPMENT.
_SPECIAL REFERENCES._
RATHKE. Zur Entwickelungsgesch. der _Blatta germanica_. Meckel’s
Arch. of Anat. u. Phys., Bd. VI. (1832).
BALFOUR. Comparative Embryology, 2 vols. (1880–1).
GRABER. Insekten, Vol. II. (1879).
LUBBOCK. Origin and Metamorphoses of Insects (1874).
KOWALEWSKY. Embryol. Studien an Würmern u. Arthropoden. Mém. Ac.
Petersb. Sér. VII., Vol. XVI. (1871).
WEISMANN. Entw. der Dipteren. Zeits. f. wiss. Zool., Bde. XIII.,
XIV. (1863–4).
METSCHNIKOFF. Embryol. Studien. an Insecten. Ib., Bd. XVI. (1866).
BÜTSCHLI. Entwicklungsgeschichte der Biene. Ib., Bd. XX. (1870).
BOBRETZKY. Bildung d. Blastoderms u. d. Keimblätter bei den
Insecten. Ib., Bd. XXXI. (1878).
NUSBAUM. Rozwój przewodów organów pteiowych u owadów (Polish).
Kosmos. (1884). [Development of Sexual Outlets in Insects.]
---- Struna i struna Leydig’a u owadów (Polish). Kosmos (1886).
[Chorda and Leydig’s chorda in Insects.]
_The Embryonic Development of the Cockroach._[174]
By JOSEPH NUSBAUM, Magister of Zoology, Warsaw.
[174] In the following description it is to be understood that the
observations have been made upon _Blatta germanica_, except where _P.
orientalis_ is expressly named.
The development of the Cockroach is by no means an easy study. It costs
some pains to find an accessible place in which the females regularly
lay their eggs, and the opaque capsule renders it hard to tell in what
stage of growth the contained embryos will be found. Accordingly,
though the development of the Cockroach has lately attracted some
observers, the inexperienced embryologist will find it more profitable
to examine the eggs of Bees, of Aphides, or of such Diptera as lay
their eggs in water.
The Cockroach is developed, like most animals, from fertilised
eggs.[175] The eggs of various animals differ much in size and form,
but always contain a formative plasma or egg-protoplasm, a germinal
vesicle (_nucleus_), and a germinal spot (_nucleolus_). Besides these
essential parts, eggs also always contain a greater or less quantity
of food-yolk, which serves for the supply of the developing embryo.
The quantity of this yolk may be small, and its granules are then
uniformly dispersed through the egg-protoplasm; or very considerable,
in which case the protoplasm and yolk become more or less sharply
defined. Eggs of the first kind are known as _holoblastic_, those of
the second kind as _meroblastic_, names suggested by the complete or
partial segmentation which these kinds of eggs respectively undergo.
When the food-yolk is very abundant it does not at first (and in some
cases does not at any time) exhibit the phenomena of growth, such as
cell-division. If, on the other hand, the yolk is scanty and evenly
dispersed through the egg-protoplasm, the segmentation proceeds
regularly and completely. The eggs of Arthropoda, including those of
the Cockroach, are meroblastic.
[175] Fertilisation consists essentially in the union of an
egg-nucleus (female nucleus) with a sperm-nucleus (male nucleus).
From this union the first segmentation-nucleus is derived.
The eggs of the Cockroach (_P. orientalis_) are enclosed (see p. 23)
sixteen together in stout capsules of horny consistence. They are
adapted to the form of the capsule, laterally compressed, convex on
the outer, and concave on the inner side. The ventral surface of
the embryo lies towards the inner, concave surface of the egg. Each
egg is provided with a very thin brownish shell (_chorion_), whose
surface is ornamented with small six-sided projections. In young eggs,
still enclosed within the ovary, the nucleus (_germinal vesicle_) and
nucleolus (_germinal spot_) can be plainly seen, but by the time they
are ready for deposition within the capsule, so large a quantity of
food-yolk, at first finely--afterwards coarsely--granular, accumulates
within them, that the germinal vesicle and spot cease to be visible.
Since the yolk of the newly-laid egg of the Cockroach is of a
consistence extremely unfavourable to hardening and microscopic
investigation, I have not been able to obtain transverse sections
of the germinal vesicle, nor to study the mode of its division
(segmentation). If, however, we may judge from what other observers
have found in the eggs of Insects more suitable for investigation than
those of the Cockroach, we shall be led to conclude that a germinal
vesicle, with a germinal spot surrounded by a thin layer of protoplasm,
lies within the nutritive yolk of the Cockroach egg. From this
protoplasm all the cells of the embryo are derived.
The germinal vesicle, together with the surrounding protoplasm,
undergoes a process of division or segmentation. Some of the cells thus
formed travel towards the surface of the egg to form a thin layer of
flattened cells investing the yolk, the so-called _blastoderm_, while
others remain scattered through the yolk, and constitute the yolk-cells
(fig. 107).
[Illustration: Fig. 104.--Ventral Plate of _Blatta germanica_, with
developing appendages, seen from below. × 20.]
On the future ventral side of the embryo (and therefore on the concave
surface of the egg) the cells of the blastoderm become columnar, and
here is formed the so-called ventral plate, the first indication of
the embryo. This is a long narrow flattened structure (fig. 104). It
is wider in front where the head segment is situated; further back it
becomes divided by many transverse lines into the primitive segments.
The total number of segments in the ventral plate of Insects is usually
seventeen.[176] Indications of the appendages appear very early. They
give rise to an unpaired labrum, paired antennæ, mandibles, and maxillæ
(two pairs). The first and second pair of maxillæ have originally,
according to Patten,[177] two and three branches respectively. Behind
the mouth-parts are found three rudimentary legs. Upon all the
abdominal segments, according to Patten, rudimentary limbs are formed;
but these soon disappear, except one pair, which persists for a time
in the form of a knobbed stalk; subsequently this, too, completely
disappears. Three or four of the hindmost segments curve under the
ventral surface of the embryo, and apparently (?) give rise to the
modified segments and appendages of the extremity of the abdomen
(fig. 105). The ventral plate lies at first directly beneath the egg
membrane (chorion), but afterwards becomes sunk in the yolk, so that
a portion of the yolk makes its way between the ventral plate and the
chorion. Whilst this portion of the yolk is perfectly homogeneous, the
remainder, placed internally to it, becomes coarsely granular, and
encloses many roundish cavities and yolk-cells. The middle region of
the body is more deeply sunk in the yolk than the two ends, and the
embryo thus assumes a curved position (fig. 105).
[176] Balfour, Embryology, Vol. I., p. 337.
[177] Q. J. Micr. Sci., Vol. XXIV., page 596 (1884).
[Illustration: Fig. 105.--Ventral Plate of _B. germanica_, side view.
× 20.]
[Illustration: Fig. 106.--Diagram to illustrate the formations of
the Embryonic Membranes. _A_, amnion; _S_, serous envelope; _B_,
blastoderm.]
This curvature of the embryo is closely connected with the formation of
the embryonic membranes. On either side of the ventral plate a fold of
the blastoderm arises, and these folds grow towards each other beneath
the chorion. Ultimately they meet along the middle line of the ventral
plate (fig. 106), and thus form a double investment, the outer layer
being the _serous envelope_, the inner the _amnion_. Between the two
the yolk passes in, as has been explained above (fig. 107).
[Illustration: Fig. 107.--Transverse section through young Embryo of
_B. germanica_. _E_, epiblast; _M_, mesoblast; _Y_, yolk-cells.]
At the same time that the embryonic membranes are forming, the
embryonic layers make their appearance. The ventral plate, which
was originally one-layered, forms the _epiblast_ or outer layer of
the embryo, and from this are subsequently derived the middle layer
(_mesoblast_) and the deep layer (_hypoblast_).
[Illustration: Fig. 108.--Diagram to illustrate the formation of the
Germinal Layers. _E_, epiblast; _M_, mesoblast.]
As to the origin of the mesoblast most observers have found[178] that
a long groove (the _germinal groove_) appears in the middle line of
the ventral plate (fig. 108), which bulges into the yolk, gradually
detaches itself from the epiblast, and completes itself into a tube.
The lumen of this tube soon becomes filled with cells, and the solid
cellular mass thus formed divides into two longitudinal tracts, which
lie right and left of the middle line of the ventral plate beneath the
epiblast, and are known as the _mesoblastic bands_. In the Cockroach
I was able to satisfy myself that in this Insect also, the mesoblast,
in all probability, arises by the formation and closure of a similar
groove of the epiblast. _M_ (fig. 108) represents the stage in which
the lumen of the groove has disappeared, and the mesoblast forms a
solid cellular mass.
[178] Kowalewsky in _Hydrophilus_, Graber in _Musca_ and _Lina_,
Patten in _Phryganidæ_, myself in _Meloe_, &c.
The origin of the hypoblast in Insects has not as yet been clearly
determined. Two quite different views on this subject have found
support. Some observers (Bobretsky, Graber, and others) maintain
that the hypoblast originates in the yolk-cells, which form a
superficial layer investing the rest of the yolk. Others (especially
Kowalewsky[179]) believe that the process is altogether different.
According to the latest observations of the eminent embryologist just
named, upon the development of the _Muscidæ_, the germinal groove
gives rise, not only to the two mesoblastic bands, but also, in its
central region, to the hypoblast. This makes its appearance, however,
not as a continuous layer, but as two hourglass-shaped rudiments, one
at the anterior, the other at the posterior end of the ventral plate.
These rudiments have their convex ends directed away from each other,
while their edges are approximated and gradually meet so as to form a
continuous hypoblast beneath the mesoblast. Although I have not been
able completely to satisfy myself as to the mode of formation of the
hypoblast in the Cockroach, I have observed stages of development which
lead me to suppose that it proceeds in this Insect in a manner similar
to that observed by Kowalewsky in _Muscidæ_. The hourglass-shaped
rudiments of the hypoblast become pushed upwards by those foldings-in
of the epiblast which form towards the anterior and posterior ends of
the embryo, and give rise to the stomodæum and proctodæum.[180]
[179] Biolog. Centrablatt. Bd. VI., No. 2 (1886).
[180] These terms are explained on p. 115.
The stage of development in which the germinal groove appears, by
the folding inwards of the epiblast, has been observed in many other
animals, and is known as the Gastræa-stage. In all higher types
(Vertebrates, the higher Worms, Arthropoda, Echinodermata) the
mesoblast and hypoblast are formed in the folded-in part of the Gastræa
in a manner similar to that observed in Insects.
The yolk-cells, which some observers have supposed to form the
hypoblast, are believed by Kowalewsky to have no other function except
that of the disintegration and solution of the yolk. I can, however,
with confidence affirm that in the Cockroach these cells take part in
the formation of permanent tissues (see below).
Each of the two mesoblastic bands which lie right and left of the
germinal groove divides into many successive somites, and each
of these becomes hollow. Every such somite consists of an inner
(dorsal) one-layered and an outer (ventral) many-layered wall, the
latter being in contact with the epiblast. The cavities of all the
somites unite to form a common cavity, the _cœlom_ or perivisceral
space of the Cockroach. The cœlom, like the cavities in which it
originates, is bounded by two layers of mesoblast--an inner, the
so-called _splanchnic_ or visceral layer, which lies on the outer
side of the hypoblast, and an outer _somatic_ or parietal layer,
beneath the epiblast. There are accordingly four layers in the
Cockroach-embryo--viz., (1) _epiblast_, from which the integument and
nervous system are developed; (2) _somatic layer of mesoblast_, mainly
converted into the muscles of the body-wall; (3) _splanchnic layer of
mesoblast_, yielding the muscular coat of the alimentary canal; and (4)
_hypoblast_, yielding the epithelium of the mesenteron.
[Illustration: Fig. 109.--Transverse sections of Embryo of _B.
germanica_, with rudimentary nervous system (Oc. 4, Obj. D.D. Zeiss).
_N_, nervous system; _M_, mesoblastic somites.]
Scattered yolk-cells associate themselves with the mesoblast cells, so
that the constituents of the mesoblast have a two-fold origin. Fig. 109
shows that the yolk-cells are large, finely granular, and provided
with many (3–6) nuclei and nucleoli. They send out many branching
protoplasmic threads, which connect the different cells together,
and thus form a cellular network. Certain cells separate themselves
from the rest, apply themselves to the walls of the somites, and form
a provisional diaphragm (fig. 110, _D_) consisting of a layer of
flattened cells;[181] other cells (fig. 109) pass into and through
the walls of the somites, and reach their central cavity, where they
increase in number and blend with the mesoblast cells. What finally
becomes of them I cannot say; perhaps they form the fat-body.
[181] Cf. Korotneff, Embryol. der Gryllotalpa. Zeits. f. wiss. Zool.
(1885).
[Illustration: Fig. 110.--Transverse section through ventral region
of Embryo of _B. germanica_. The nerve-cord has by this time detached
itself from the epiblast, _E_. _D_ is the temporary diaphragm; _Ch_,
temporary cellular band, from which the neurilemma proceeds; _Ap_,
appendages in section; _M_, mesoblast; _N_, nerve-cord. (Oc. 4. Obj.
BB. Zeiss).]
The ventral plate occupies, as I have explained, the future ventral
surface of the Insect, and here only at first both the embryonic
membranes are to be met with. On the sides and above the yolk is
invested by the serous envelope alone. The ventral plate, however,
gradually extends upwards upon the sides of the egg, in the directions
of the arrows (fig. 107), and finally closes upon the dorsal surface of
the embryo, so as completely to invest the whole yolk. Every segment
of the embryo shows at a certain stage numerous clusters of spherical
granules, which according to Patten (loc. cit.) are composed of urates
(fig. 111, _S_).
We shall now proceed to consider the development of the several organs
of the Cockroach.
[Illustration: Fig. 111.--Transverse section of older Embryo of _B.
germanica_ (abdomen). _E_, Epiblast; _H_, hypoblast; _Ht_, heart; _G_,
reproductive organs; _S_, spherical granules.]
_Nervous System._--Along the middle line of the whole ventral
surface there is formed a somewhat deep groove-like infolding of the
epiblast, bounded on either side by paired solid thickenings, which
detach themselves from the epiblast (fig. 110, _N_) and constitute
the double nervous chain. In many other Insects a median cord (from
which are derived the transverse interganglionic commissures) forms
along the bottom of the nervous fold. This secondary median fold is
very inconspicuous and slightly developed in the Cockroach, so that
the transverse commissures between the developing ganglia are mainly
contributed by the cellular substance of the lateral nervous band.
The brain is formed out of two epiblastic thickenings which occupy
shallow depressions. The so-called _inner neurilemma_, which surrounds
the ventral nerve-cord, is developed as follows:--Along the ventral
nerve-cord, and between its lateral halves, a small solid cellular
band (fig. 110, _Ch_) is developed out of the mesoblastic diaphragm
described above. This grows round the ventral nerve-cord on all sides
(fig. 112, _N_′), passing also inwards between the central fibrillar
tract and the outer cellular layer, and thus forming the thin
membrane which invests the central nervous mass (fig. 112, _N_″). The
above-mentioned solid mesoblastic band, which exists for a very short
time only, may perhaps be homologised with the chorda dorsalis of
Vertebrates, and the chorda of the higher Worms, since in these types
also the chorda forms a solid cellular band of meso-hypoblastic origin,
lying between the nervous system and the hypoblast. The peripheral
nerves arise as direct prolongations of the fibrillar substance of the
nerve-cord.
[Illustration: Fig. 112.--Transverse section of Nerve-cord of Embryo
of _B. germanica_ (Oc. 4, Obj. D.D. Zeiss). _C_, cellular layer; _F_,
fibrillar substance (_punkt-substance_ of Leydig); _Ch_, cellular band;
_N_′ _N_″ inner and outer neurilemma.]
[Illustration: Fig. 113.--Alimentary Canal of Embryo of _B. germanica_.
Copied from Rathke, loc. cit., but differently lettered. _st_,
stomodæum, already divided into œsophagus, crop, and gizzard; _m_,
mesenteron; _pr_, proctodæum, with Malpighian tubules (removed on the
right side). × 12.]
_Alimentary Canal._--The epithelium of the mesenteron is formed out of
the hypoblast, whose cells assume a cubical form and gradually absorb
the yolk. The epithelium of the stomodæum and proctodæum is derived,
however, from two epiblastic involutions at the fore and hind ends of
the embryo. The muscular coat of the alimentary canal is contributed by
the splanchnic layer of the mesoblast. The mesenteron in an early stage
of development appears as an oval sac of greenish colour (fig. 113),
faintly seen through the body-wall. The cæcal tubes are extensions
of the mesenteron, the Malpighian tubules of the proctodæum. The
epiblastic invaginations may be recognised in all stages of growth by
their chitinous lining and layer of chitinogenous cells, continuous
with the similar layers in the external integument.
_Tracheal System._--Tubular infoldings of the epiblast, forming at
regular intervals along the sides of the embryo and projecting into the
somatic mesoblast, give rise to the paired tracheal tubes, which are at
first simple and distinct from one another.[182]
[182] In _Gryllotalpa_ (Dohrn), as in Spiders, some Myriopods
and _Peripatus_ (Moseley, Phil. Trans., 1874), each stigma, with
its branches, constitutes throughout life a separate system. The
salivary glands arise in the same way, not, like the salivary
glands of Vertebrates, as extensions of the alimentary canal, but
as independent pits opening behind the mouth. Both the tracheal and
the salivary passages are believed to be special modifications of
cutaneous glands (Moseley).
_Heart._--The wall of the heart in Insects is of mesoblastic origin,
and develops from paired rudiments derived from that peripheral part of
each mesoblastic band which unites the somatic to the splanchnic layer.
In this layer two lateral semi-cylindrical rudiments appear, which, as
the mesoblastic bands meet on the dorsal surface of the embryo, are
brought into contact and unite to form the heart (fig. 111). The heart
is therefore hollow from the first, its cavity not being constricted
off from the permanent perivisceral space enclosed by the mesoblast,
but being a vestige of the primitive embryonic blastocœl, which is
bounded by the epiblast, as well as by the two other embryonic layers.
Such a mode of the development of the heart was observed by Bütschli in
the Bee, and by Korotneff in the Mole Cricket. I am convinced, from my
own observations, that the heart of the Cockroach originates in this
way, though it is to be observed that, in consequence of Patten’s
results,[183] the question requires further investigation. According
to Patten the mesoblastic layers of the embryo pulsate rhythmically
long before the formation of the heart. Patten also states that the
blood-corpuscles are partially derived from the wall of the heart.
[183] Loc. cit.
[Illustration: Fig. 114.--Young Ovary of _B. germanica_. (Oc. 2, Ob.
DD, Zeiss.)]
[Illustration: Fig. 115.--Young Testis of _B. germanica_. (Oc. 2, Ob.
DD, Zeiss.)]
[Illustration: Figs. 116, 117, 118.--Three stages of development of
tegumentary portion of Male Sexual Organs of _P. orientalis_. (Oc. 1,
Ob. BB, Zeiss.) _VD_, vas deferens; _VS_, vesicula seminalis; _D_,
ductus ejaculatorius; _P_, _p_, penis and its lateral appendages.]
_Reproductive Organs._--In _P. orientalis_ the reproductive organs
are developed as follows:--The reproductive glands have a mesoblastic
origin. The immature ovaries and testes take the form of elongate oval
bodies, which prolong themselves backwards into a long thin thread-like
cord or ligament (figs. 114, 115). These lie in the perivisceral space,
between the somatic and splanchnic layers of the mesoblast, and on the
sides of the abdomen. The glands divide tolerably early into chambers,
which have, however, a communicating passage (figs. 114, 115). From
their backward-directed prolongations arises the epithelium of the vasa
deferentia and oviducts. All other parts of the reproductive ducts are
developed out of tegumentary thickenings of the ventral surface in
the last abdominal segment, and the last but one. These thickenings
are at first paired,[184] but afterwards blend to form single organs
(fig. 118). Within the tegumentary thickenings just described, there
appear in the male Cockroach two anterior closed cavities which unite
to form the single cavity of the permanent mushroom-shaped body
(_vesicula seminalis_). A posterior cavity becomes specialised as
the ductus ejaculatorius, while the hindmost part of the thickening,
which is at first double, afterwards by coalescence single, forms
the penis (figs. 117, 118). The accessory reproductive glands have
also a tegumentary origin. In the female Cockroach the chitinogenous
epithelium of the integument gives rise to the uterus, vagina,[185]
and accessory glands, the muscular and connective tissue layers of the
sexual apparatus being formed out of loose mesoblastic cells.[186]
Joseph Nusbaum.
[184] This arrangement persists only in _Ephemeridæ_ among Insects
(Palmen, Ueb. paarigen Ausführungsgänge der Geschlechtsorgane bei
Insekten, 1884).
[185] Genital pouch of the preceding description.
[186] Indications, which we have not found time to work out, lead us
to think that the development of the specially modified segments and
appendages in the male and female Cockroach needs re-examination. We
hope to treat this subject separately on a future occasion.--L. C. M.
and A. D.
_Post-embryonic Development._
At the time of hatching the Cockroach resembles its parent in all
essentials, the wings being the only organs which are developed
subsequently, not as entirely new parts, but as extensions of the
lateral edges of the thoracic terga. The mode of life of the young
Cockroach is like that of the adult, and development may be said to
be direct, or with only a trifling amount of metamorphosis. In the
Thysanura even this small post-embryonic change ceases to appear,
and the Insect, when it leaves the egg, differs from its parent
only in size. It is probable that development without metamorphosis
was once the rule among Insects. At present such is by no means the
case. Insects furnish the most familiar and striking, though, as will
appear by-and-by, not the most typical examples of development with
metamorphosis. In many text-books the quiescent pupa and the winged
imago are not unnaturally described as normal stages, which are
exceptionally wanting in Orthoptera, Hemiptera, Thysanura, and other
“ametabolous” Insects. It is, however, really the “holometabolous”
Insects undergoing what is called “complete metamorphosis,” which are
exceptional, deviating not only from such little-specialised orders as
Thysanura and Orthoptera, but from nearly all animals which exhibit a
marked degree of metamorphosis. We shall endeavour to make good this
statement, and to show that the Cockroach is normal in its absence of
conspicuous post-embryonic change, while the Butterfly, Bee, Beetle,
and Gnat are peculiar even among metamorphic animals.
_Animal Metamorphoses._
To investigate the causes of metamorphosis, let us select from the
same sub-kingdom two animals as unlike as possible with respect to the
amount of post-embryonic change to which they are subject. We can find
no better examples than Amphioxus and the Chick.
The newly-hatched Amphioxus is a small, two-layered, hollow sac, which
moves through the sea by the play of cilia which project everywhere
from its outer surface. It is a Gastræa, a little simpler than the
Hydra, and far simpler than a Jelly-fish. As yet it possesses no
nervous system, heart, respiratory organs, or skeleton. The most
expert zoologist, ignorant of its life-history, could not determine
its zoological position. He would most likely guess that it would turn
either into a polyp or a worm.
The Chick, on the other hand, at the tenth day of incubation, is
already a Bird, with feathers, wings, and beak. When it chips the shell
it is a young fowl. It has the skull, the skeleton, the toes, and the
bill characteristic of its kind, and no child would hesitate to call it
a young Bird.
Amphioxus is, therefore, a Vertebrate (if for shortness we may so name
a creature without vertebræ, brain, or skull), which develops with
metamorphosis, being at first altogether unlike its parent. The Chick
is a Vertebrate which develops directly, without metamorphosis. Let us
now ask what other peculiarities go with this difference in mode of
development.
Amphioxus produces many small eggs (1/10 mm. in diameter) without
distinct yolk, and consequently segmenting regularly. The adult is of
small size (2 to 3 in. long), far beneath the Chick in zoological rank,
and of marine habitat.
The Fowl lays one egg at once, which is of enormous size and provided
with abundant yolk, hence undergoing partial segmentation. The Fowl is
much bigger than Amphioxus, much higher in the animal scale, and of
terrestrial habitat.
Which of the peculiarities thus associated governs the rest? Is it the
number or size of the eggs? Or the size, zoological rank, or habitat of
the adult? The question cannot be answered without a wider collection
of examples. Let us run over the great divisions of the Animal
Kingdom, and collect all the facts which seem to be significant. We may
omit the Protozoa, which never develop multicellular tissues, and in
which segmentation and all subsequent development are therefore absent.
PORIFERA (Sponges).--Nearly all marine and undergoing metamorphosis,
the larva being wholly or partially ciliated.
CŒLENTERATES undergo metamorphosis, the immediate product of the
ovum being nearly always a _planula_, or two-layered hollow sac,
usually devoid of a mouth, and moving about by external cilia. In
many Cœlenterates the complicated process of development known as
Alternation of Generations occurs. The sedentary Anemones pass
through a planula stage, but within the body of the parent. Among the
few Cœlenterates which have no free planula stage is the one truly
fluviatile genus--Hydra.
WORMS are remarkable for the difference between closely allied
forms with respect to the presence or absence of metamorphosis. The
non-parasitic freshwater and terrestrial Worms, however (_e.g._,
Earthworms, Leeches, all freshwater Dendrocœla, and Rhabdocœla), do not
undergo metamorphosis. In the parasitic forms complicated metamorphosis
is common, and may be explained by the extraordinary difficulties often
encountered in gaining access to the body of a new host.
All POLYZOA are aquatic (fluviatile or marine), and all produce
ciliated embryos, unlike the parent.
BRACHIOPODA are all marine, and produce ciliated embryos.
ECHINODERMS usually undergo striking metamorphosis, but certain
viviparous or marsupial forms develop directly. There are no fluviatile
or terrestrial Echinoderms.
LAMELLIBRANCHIATE MOLLUSCA have peculiar locomotive larvæ, provided
with a ring of cilia, and usually with a long vibratile lash. These
temporary organs are reduced or suppressed in the freshwater forms.
There are no terrestrial Lamellibranchs.
SNAILS have also a temporary ciliated band, but in the freshwater
species it is slightly developed (_Limnæus_), and it is totally wanting
in the terrestrial _Helicidæ_.
CEPHALOPODA, which are all marine, have no ciliated band, and the
post-embryonic changes do not amount to metamorphosis. There is usually
a much larger yolk-sac than in other Mollusca.
CRUSTACEA usually pass through well-marked phases. _Peneus_ presents
five stages of growth (including the adult), the earlier being common
to many lower Crustacea. The Crab passes through three, beginning with
the third of _Peneus_; the Lobster through two; while the freshwater
Crayfish, when hatched, is already in the fifth and last.
FISHES seldom undergo any post-embryonic change amounting to
metamorphosis. _Amphioxus_ (if _Amphioxus_ be indeed a fish) is the
only well-marked case.
AMPHIBIA develop without conspicuous metamorphosis, except in the case
of the Frogs and Toads (Anura), which begin life as aquatic, tailed,
gill-bearing, and footless tadpoles.
REPTILES, BIRDS, and MAMMALS do not undergo transformation.
This survey, hasty as it necessarily is, shows that habitat is
a material circumstance. Larval stages are apt to be suppressed
in fluviatile and terrestrial forms. Further, it would seem that
zoological rank is not without influence. Metamorphosis is absent in
Cephalopoda, the highest class of Mollusca, and in all but the lowest
Vertebrates, while it is almost universal in Cœlenterates, Echinoderms,
and Lamellibranchs.
It has often been remarked that the quantity of food-yolk indicates
the course of development. If a large store of food has been laid up
for the young animal, it can continue its growth without any effort
of its own, and it leaves the egg well equipped for the battle of
life. Where there is little or no yolk, the embryo is turned out in an
ill-furnished condition to seek its own food. This early liberation
implies metamorphosis, for the small and feeble larva must make use of
temporary organs. Some very simple locomotive appendages are almost
universally needed, to enable it to get away from the place of its
birth, which is usually stocked with as much life as it can support.
Some animals, therefore, are like well-to-do people, who can provide
their children with food, clothes, schooling, and pocket-money. Their
fortunate offspring grow at ease, and are not driven to premature
exercise of their limbs or wits. Others are like starving families,
which are forced to send their children to sell matches or newspapers
in the streets. It is a question of the amount of capital or
accumulated food which is at command.
The connection between zoological rank and the absence of metamorphosis
is also explained by what we see among men. High zoological position
ordinarily implies strength or intelligence, and the strong and knowing
can do better for their offspring than the puny and sluggish. It does
not cost a Shark or a quadruped too much to hatch its young in its
own body, while Spiders and Earwigs,[187] which are among the highest
Invertebrates, defend their progeny, as do Mammals and Birds, the
highest Vertebrates.
[187] It may be useful to point out the following examples of
parental care among animals in which, as a rule, the eggs are left
to take care of themselves. It will be found that in general this
instinct is associated with high zoological rank (best exemplified
by Mammals and Birds), land or freshwater habitat, reduced number of
eggs, and direct development.
AMPHIBIA.--The eggs are sometimes hatched by the male (_Alytes
obstetricans_, _Rhinoderma Darwinii_), or placed by the male in
pouches on the back of the female (_Pipa dorsigera_, _Notodelphis
ovifera_, _Nototrema marsupiatum_), or carried during hatching by the
female (_Polypedates reticulatus_).
FISHES.--The Stickleback and others build nests. Of eleven genera of
nest-building Fishes, eight are freshwater. The number of eggs is
unusually small. Many Siluroids have the eggs hatched in the mouth
of the males, a few under the belly of the female. The species are
both marine and freshwater, the eggs few and large. Lophobranchiate
fishes usually have the eggs hatched by the male. They are marine;
the eggs few and large. Many sharks hatch their eggs, which are very
few, within the body. _Mustelus lævis_ has a placenta formed out of
the yolk-sac.
INSECTS.--De Geer has described the incubation of the Earwig, and the
care of the brood by the female. The cases of the social Hymenoptera,
&c., are universally known.
SPIDERS.--The care of the female spider for her eggs is well known.
CRUSTACEANS.--The Crayfish hatches and subsequently protects her
young. _Mysis_, _Diastylis_ (_Cuma_), and some Isopods hatch their
eggs. _Gammarus locusta_ is followed about by her brood, which
shelter beneath her when alarmed. _Podocerus capillatus_ builds a
nest among corallines. Several of the _Caprellidæ_ hatch or otherwise
protect their young. All these, except the Crayfish, are marine; the
eggs commonly fewer than usual.
ECHINODERMS.--Many cases of “marsupial development” have been
recorded in the species of the Southern seas. Here development,
contrary to the rule in Echinodermata, is direct.
But what has all this to do with habitat? Are fluviatile and
terrestrial animals, as a rule, better off than marine animals?
Possibly they are. In the confined and isolated fresh waters at
least, the struggle for existence is undoubtedly less severe than in
the waters of the sea. This is shown by the slow rate of change in
freshwater types. Many of our genera of land and freshwater shells
date back at least as far as to Purbeck and Wealden times, while our
common pond-mussel is represented in the Coal Measures. The comparative
security of fresh waters is probably the reason why so many marine
fishes enter rivers to spawn.
More important, and less open to question, is the direct action of the
sphere of life. The cheap method of turning the embryo out to shift
for itself can seldom be practised with success on land. But in water
floating is easy, and swimming not difficult. A very slightly-built
larva can move about by means of cilia, and a whole brood can disperse
far and wide in search of food, while still in a mere planula
condition--hollow sacs, without mouth, nerves, or sense-organs.
Afterwards the little locomotive larva settles down, opens a mouth, and
begins to feed. Nearly the whole of its development is carried on at
its own charge.
The extra risks to which marine animals are exposed also tell in favour
of transformation, for they are met by an increase in the number of
ova. Marine species commonly lay more eggs than freshwater animals of
like habits. The Cod is said to produce nine million eggs; the Salmon
from twenty to thirty thousand; the Stickleback only about one hundred,
which are guarded during hatching by the male. The Siluroid fish,
_Arius_, lays a very few eggs, as big as small cherries, which the male
carries about in his mouth.
Without laying stress upon such figures as these, which cannot be
impartially selected, we can safely affirm that marine forms are
commonly far more prolific than their freshwater allies. But high
numbers increase the difficulty of providing yolk for each, and
thus tend to early exclusion, and subsequent transformation. We may
rationally connect marine habitat with small eggs, poorly supplied with
yolk, segmenting regularly, and producing larva which develop with
metamorphosis.
In fresh waters dispersal can seldom be very effective. The area is
usually small, and communicates with other freshwater basins only
through the sea. Migration to a considerable distance is usually
impossible, and migration to a trifling distance useless. Moreover,
competition is not too severe to prevent some accumulation of food by
the parent on behalf of the family.
On land the conditions are still less favourable to larval
transformation. Very early migration is altogether impossible. Any kind
of locomotion by land implies muscles of complicated arrangement, and,
as a rule, there must be some sort of skeleton to support the weight
of the body. The larva, if turned out in a Gastræa condition would
simply perish without a struggle.[188] Nor is great precocity needful.
The terrestrial animal is commonly of complicated structure, active,
and well furnished with means of information. It can lay-by for its
offspring, and nourish them within its own body, or at least by food
stored up in the egg.
[188] The minute and early larvæ of _Tœnia_ and _Distomum_ may appear
to contradict this statement. They really inhabit the film of water
which spreads over wet grass, though they are capable of enduring dry
conditions for a short time, like Rotifers and many Infusoria.
The influence of habitat upon development may be recapitulated as
follows:--
MARINE HABITAT.--Eggs many. Yolk small. Segmentation often regular.
Young hatched early. Development with metamorphosis. [The most
conspicuous exceptions are Cephalopoda and marine Vertebrata.]
FLUVIATILE HABITAT.--Eggs fewer. Yolk larger. Segmentation often
unequal. Young hatched later. Development direct, or with late
metamorphosis only. [The most obvious exceptions are Frogs and Toads,
which develop with metamorphosis.]
TERRESTRIAL HABITAT.--Eggs few. Yolk large [except where the young are
supplied by maternal blood]. Segmentation often partial. Young hatched
late. Development without metamorphosis. [An exception is found in
Insects, which usually exhibit conspicuous metamorphosis, though the
yolk is large, and the type of segmentation partial or unequal.]
Let us now take up the exceptions, and see whether these are capable of
satisfactory explanation.
1.--Cephalopoda and marine Vertebrates, unlike other inhabitants of
the sea, develop without metamorphosis. But these are large animals of
relatively high intelligence, well able to feed and protect their young
until development is completely accomplished.
2.--Frogs and Toads, unlike other fluviatile animals, develop with
metamorphosis. The last and most conspicuous change, however, from
the gill-bearing and tailed tadpole to the air-breathing and tailless
frog, hardly belongs to the ordinary period of embryonic development.
When the tadpole has four limbs and a long tail it has already
reached the point at which the more primitive Amphibia (_Menopoma_,
_Proteus_, &c.) become sexually mature. The loss of the tail, the
lengthening of the hind limbs, and the complete adaptation to pulmonary
respiration, relate to the mode of dispersal of the adult. Cut off
from early dispersal by the isolation of their breeding-places and
the difficulty of land migration, Frogs migrate from pool to pool as
full-grown animals. The eggs are thus laid in new sites, and very small
basins--ditches and pools which dry up in summer--can be used for
spawning. To this peculiar facility in finding new spawning grounds
the Anura no doubt owe their success in life, of which the vast number
of nearly-allied species furnishes an incontrovertible proof. But the
adaptation to terrestrial locomotion necessarily comes late in life,
after the normal and primitive adult Amphibian condition has been
attained. It is by a _secondary adult metamorphosis_ that the aquatic
tadpole turns into the land-traversing frog. The change is not fairly
comparable to any process of development by which other animals gain
the adult structure characteristic of their class and order, but (in
respect of the time of its occurrence) resembles the late assumption of
secondary sexual characters, such as the antlers of the stag, or the
train of the peacock.
3.--Lastly, we come to the exceptional case of Insects which, unlike
other terrestrial animals, develop with metamorphosis. The Anurous
Amphibia have prepared us to recognise this too as a case of secondary
adult (post-embryonic) metamorphosis. Thysanuran or Orthopterous larvæ
cannot differ very widely from the adult form of primitive Insects.
From wingless, hexapod Insects, like Cockroach larvæ in all essentials
of external form, have been derived, on the one hand, the winged imago,
adapted in the more specialised orders to a brief pairing season
exclusively spent in migration and propagation; on the other hand, the
footless maggot or quiescent pupa.
Insects, like Frogs, disperse as adults, because of the difficulty
of the medium, aerial locomotion being even more difficult than
locomotion by land, and implying the highest muscular and respiratory
efficiency. The flying state is attained by a late metamorphosis, which
has not yet become universal in the class, while it is not found in
other Tracheates at all. _Peripatus_, Scorpions, and Myriopods become
sexually mature when they reach the stage which corresponds to the
ordinary less-modified Insect-nymph, with segmented body, walking legs,
and mouth-parts resembling those of the parent.[189]
[189] It is possible that the curious cases of agamogenetic
reproduction of the larvæ of _Aphis_, _Cecidomyia_, and _Chironomus_
are vestiges of the original fertility of Insect larvæ.
The Caterpillar is not, as Harvey[190] maintained, a kind of walking
egg; it is rather the primitive adult Tracheate modified in accordance
with its own special needs. It may be sexually immature, imperfect,
destined to attain more elaborate development in a following stage, but
it nevertheless marks the stage in which the remote Tracheate ancestor
attained complete maturity. Where it differs from the primitive form,
hatched with all the characters of the adult, the changes are adaptive
and secondary.[191]
[190] “Alia vero semen adhuc imperfectum et immaturatum recludunt,
incrementum et perfectionem, sive maturitatem, soris acquisiturum;
ut plurima genera piscium, ranæ, item mollia, crustata, testacea, et
cochleæ: quorum ova primum exposita sunt, veluti origines duntaxat,
inceptiones et vitelli; qui postea albumina sibi ipsis circum
circa induunt; tandemque alimentum sibi attrahentes, concoquentes
et apponentes, in perfectum semen atque ovum evadunt. Talia sunt
insectorum semina (vermes ab Aristotele dicta) quæ initio imperfecte
edita sibi victum quærunt indeque nutriuntur et augentur, de eruca
in aureliam; de ovo imperfecto in perfectum ovum et semen.”--_De
generatione_, Exc. II., p. 183 (1666). Viallanes justifies this view
by applying it to the histolysis and regeneration of the tissues in
Diptera. But these remarkable changes are surely secondary, adaptive,
and peculiar, like the footless maggot itself, whose conversion into
a swift-flying imago renders necessary so complete a reconstruction.
[191] The reader is recommended to refer to Fritz Müller’s Facts and
Arguments for Darwin, especially chap. xi.; to Balfour’s Embryology,
Vol. II., chap. xiii., sect. ii.; and to Lubbock’s Origin and
Metamorphoses of Insects.
_The Genealogy of Insects._
To construct from embryological and other data a chart of the descent
of Insects, and of the different orders within the class, is an
attempt too hazardous for a student’s text-book.[192] A review of
the facts of Arthropod development led Balfour[193] to conclude that
the whole of the Arthropoda cannot be united in a common phylum. The
Tracheata are probably “descended from a terrestrial Annelidan type
related to _Peripatus_.... The Crustacea, on the other hand, are
clearly descended from a Phyllopod-like ancestor, which can be in no
way related to _Peripatus_.” The resemblances between the Arthropoda
appear therefore to be traceable to no nearer common ancestors than
some unknown Annelid, probably marine, and furnished with a chitinous
cuticle, an œsophageal nervous ring, and perhaps with jointed
appendages. Zoological convenience must give place to the results of
embryological and historical research, and we shall probably have
to reassign the classes hitherto grouped under the easily defined
sub-kingdom of Arthropoda.
[192] Those who care to see a bold experiment of this kind may refer
to Haeckel’s Schöpfungsgeschichte.
[193] Comp. Embryology, Vol. I., p. 451.
Sir John Lubbock has explained, in his very interesting treatise on
the Origin and Metamorphoses of Insects, the reasons which lead him
to conclude “that Insects generally are descended from ancestors
resembling the existing genus _Campodea_ [sub-order Collembola],
and that these again have arisen from others belonging to a type
represented more or less closely by the existing genus _Lindia_” [a
non-ciliated Rotifer].
Present knowledge does not, therefore, justify a more definite
statement of the genealogy of Insects than this, that in common with
Crustacea they had Annelid ancestors, and that _Lindia_, _Peripatus_,
and _Campodea_ approximately represent three successive stages of the
descent. When we reflect that Cockroaches themselves reach back to
the immeasurably distant palæozoic epoch, we get some misty notion of
the antiquity and duration of those still remoter ages during which
Tracheates, and afterwards Insects, slowly established themselves as
new and distinct groups of animals.