1216 
SCANDINAVIAN FISHES. 
enveloped in a transparent membrane, free in the water, 
where they are impregnated. When one of these eggs 
has undergone its total segmentation, it hollows itself 
out — it becomes a vesicle or rather an entirely closed 
sac (tig. 371, E), the cavity of which answers to the 
segmentation cavity (under the blastoderm) in the ova 
of the higher vertebrates and, like the said cavity, is 
principally obliterated, the sac being flattened ( E ), so 
that the lower wall comes nearer to the upper and at 
last lays itself close up to the same ( F ). The original 
sac has now become a bowl; and the rim hereof is 
contracted into a narrower and narrower aperture ( Gm ). 
Meanwhile the external layer of the bowl ( Ekt ) has 
developed small mobile bristles (flagella) on its cells, 
one to each cell (Germ. Geisselcell ): — the egg has at- 
tained a stage of development common to most of the 
invertebrates and called by Haeckel the gastrula. Now 
it dances briskly about within its egg-shell (vitelline 
Fig. 372. A', an embryo, prolongated and with the rudiments of the 
first proto vertebra} (one on each side of the intestinal canal) seen from 
the dorsal side. After Hatschek. X 77. A", transverse section of a 
similar embryo. After Kowalewsky. A"’, optical longitudinal section 
through the middle of A'. After Hatschek. X 140. B ’ , a larva with 
two pairs of protovertebrm, seen from the dorsal side. After Hat- 
schek. X 77. B ", optical longitudinal section through the middle 
of a similar larva. After Hatschek. X 140. 
At the stage A (after Hatschek) the embryo quits the egg by 
bursting the membrane thereof. The gastrula mouth is still present, but 
is overgrown from below (see B") by the ectoderm, which has besides 
raised itself on the sides of the back, thus forming a wide groove (n), 
the edges of which (dorsal ridges, r) grow more and more together to 
form a canal (the future central canal of the spinal cord; cf. nc in fig. 
380). In A the first protovertebra ( 1 — on each side a vesicular dilatation 
of the entoderm) has begun to differentiate itself, after the appearance 
between the first two germinal layers (ectoderm, Ekt and entoderm, 
Eiit) of the middle (third) germinal layer (mesoderm, fig. A", con- 
sisting of the animal (a) and the vegetative ( v ) lamina). According 
to Hatschek the mesoderm is developed from the entoderm, in the 
hindmost part of which two cells, larger than the rest, indicate the 
posterior limit of the mesodermal growth and have therefore been 
named the polar cells of the mesoderm (mp). Simultaneously with 
the formation of the mesoderm and its division into an outer (animal) 
and an inner (vegetative) lamina, a body-cavity (coelom, cl) has arisen 
round the intestinal canal. The lumen (internal space) of the latter 
is indicated by i. In B the rudiments of two pairs of protovertebree 
(/ and 2) have appeared. 
membrane). The two layers of which the body of this 
gastrula is composed correspond to the first two ger- 
minal layers of the higher vertebrates. The structure 
is the same, though the manner of life is widely dif- 
ferent, for this gastrula has no vitellus on which to 
rest, or from which to derive its nourishment. On the 
other hand, it has a stomach, which it has acquired by 
the invagination of the lower (eventually inner) part of 
its wall ( Ent ). In the embryos of the highest verteb- 
rates the intestinal canal indeed has quite a different 
appearance — though fundanentally constructed on the 
same principle — and is not formed until after a con- 
siderable alteration of the original germ; but in the 
batrachians the development of the said canal calls to 
mind the above-mentioned invagination, although the 
Fig. 373. Two optical longitudinal sections of a larva with 9 proto- 
vertebrge (somites, 1 — 9 ), seen from the left {A) and from the dorsal 
side ( B ). After Hatschek. X 140. The cerebrospinal canal (n) has 
now been formed and is separated from the ectoderm, but in front 
(at hy) open and behind (through the original gastrula mouth, Gin ) 
continuous with the intestinal canal (i). This has dilated anteriorly 
to the right ( dvd ) and the left ( dvs ). Under the cerebrospinal canal 
the notochord ( ch ) has appeared, and in the protovertebrm muscle cells 
begin to develop. In B it appears that the protovertebrae are set 
obliquely opposite each other, the right a little further back than the 
left; but the anterior lateral dilatations of the intestinal canal are 
still of equal size (symmetrical). 
batrachian embryo, like the mammalian, has been sup- 
plied by the transformations of the egg with a kind 
of nutritive yolk under it. 
The gastrula of the future Branchiostoma elongates 
its form of body (fig. 372), thus acquiring an anterior 
and a posterior end. In the latter lies its mouth. Soon 
it acquires a dorsal and a ventral side, for on the 
former a primitive groove appears, which becomes a 
cerebrospinal canal, following the same manner of deve- 
