6o8 



PHYLUM CHORDATA : CLASS PISCES — FISHES 



At the embryonic end the outer layer or ectoderm under- 

 goes a shght invagination (Fig. 343, x.), beginning to form 

 the roof of the future gut (g.). This inflected arc of the 

 blastoderm corresponds to the blastopore or mouth of the 

 gastrula, which is much disguised by the presence of a large 



quantity of yolk. As the 

 invagination proceeds, the 

 segmentation cavity is 

 obliterated. The floor of 

 the gut is formed by infold- 

 ing of the lateral walls. 



Along the mid-dorsal line 

 of the ectoderm a medullary 

 groove appears — the be- 

 ginning of the central 

 nervous system. Its sides 

 afterwards arch towards one 

 another, and meet to form 

 a medullary canal (Fig. 343, 

 n.c). A posterior com- 

 munication between this 

 dorsal nervous tube above 

 and the ventral alimentary 

 tube persists for some time 

 as the neurenteric canal 



(Fig. 343> ^i^-^-)' 



The mesoderm arises as 



two lateral plates, one on 

 each side of the medullary 

 groove. The plates seem 

 to arise as a pair of solid 

 outgrowths from the wall 

 of the gut. They are after- 

 wards divided into seg- 

 ments. Between the mesoderm plates, along the mid- 

 dorsal line of the gut, the notochord is established (Fig. 



343, n.). . . c 



Besides the internal establishment and differentiation or 

 layers, there are two important processes — (a) the growth 

 of the blastoderm around the yolk, (b) the folding off of 

 the embryo from the yolk. The result of the two processes 



Fig. 344. — Embryo dogfish in egg- 

 case ("■ mermaid's purse ") which 

 has been cut open to show con- 

 tents. 



e.g., " External " gills ; d.f., dorsal fin 

 fold ; y.s., yolk-sac ; St., stalk of yolk- 

 sac ; T., tendrils, prolongations of egg- 

 case by means of which it is moored to 

 seaweed, an " adaptation " in a dead 

 substance. 



