1216 



SCANniNAVIAN FISHKS. 



enveloped in a transparent nieniln-ane, 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 <jf which answers to tlu' 

 segmentation cavity (under the blastoderm) in tlie ova 

 of the higher vertebrates and, like the said cavity, is 

 principally obliterated, the sac being flattened (E), so 

 tjiat tlie lower wall comes nearer to the upper and at 

 last lays itself close up to tlie same (F). The original 

 sac has now become a bowl; and the rim hereof is 

 contracted into a narrower and narrower aperture (Gin). 

 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 H^eckel the gastrula. Now 

 it dances briskly about within its egg-shell (vitelline 



Fig. 372. A', an embryo, prolongated and witli llie rudiments of tlie 

 first protovertebrfE (one on each side of the intestinal canal) seen from 

 the dorsal side. After Hatsohek. X 77. .4", transverse section of a 

 similar embryo. After Kowalewsky. ,4'", optical longitudinal section 

 through the middle of A'. After Hatschek. X 140. B' . a larva with 

 two pairs f.f protovertebrje, seen from the dorsal side. After Hat- 

 SOHKK. X 77. B", optical longitudinal section through the mi<idle 

 of a similar larva. After Hatschek. X 140. 



.\t 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 (h), 

 the edges of which (dorsal ridges, r) grow more and uiore together to 

 form a canal (the future central canal of the spinal cord; cf. nc in fig. 

 .380). In ^4 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, Kkt and entoderm, 

 Enl) of the middle (third) germinal layer (mesoderm, fig. A", con- 

 sisting of the animal (a) and the vegetative (y) 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 

 nanii'il the pnhir cells of the mesoderm (iiip). Sinmltaneoiisly with 

 thi' t'lirniiiiion of tlie mesoderm and its division into an onter (animal) 



and an inner (vegetative) lamina, a body-cavity (ccelom, 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 protovertebrae 

 (/ 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 iiigher vertebrates. The structure 

 is the same, though the manner of life is widelv dif- 

 ferent, for this gastrula has no vitellus on wliich 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 (Enf). In the embryos of the highest verteb- 

 rates the intestinal canal indeed has cjuite a different 

 appearance — though fundanentally constructed on the 

 .same principle — and is not formed until after a con- 

 siderable alteration of the original germ; l)ut 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- 

 vertebrje (somites, / — .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 Ay) open and behind (through the original gastrula mouth, Gm) 

 continuous with the intestinal canal (i). This has dilated anteriorly 

 to the right (dvd) and the left (dcs). Under the cerebrospinal canal 

 the notochord (ch) has appeared, and in the protovertebrae muscle cells 

 begin to develop. In B it appears that the protovertebrae are set 

 obliquely opposite eacli (■ther, tlie right a little further back than the 

 left; but the anterior lateral dilatations of the intestinal canal are 

 still of oi|TiaI size (symmetrical). 



batrachian embryo, like the mammalian, has l)een 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 (tig. .S7'2), 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- 



