194 ESSENTIALS OF ZOOLOGY 



covered externally by ectoderm, for we may now presume 

 that the ectoderm has advanced completely over the yolk. 

 The endoderm presents on its roof the notochord, which is 

 narrowed and separated. With the formation of the neur- 

 enteric canal the growth of the nerve cord, of the notochord, 

 of the mesoderm, as of the covering ectoderm and the enteric 

 endoderm, proceeds directly from the cells, constantly multi- 

 plying in the neighbourhood of the neurenteric canal. 



The fate of these structures has already been pointed out 

 in the foregoing account. It will be plain that as the embryo 

 increases in size the yolk appears smaller in comparison, 

 forming a sac depending from the ventral side of the embryo. 

 It is called the yolk sac, and the attachment to the body, 

 which is narrowed, is called the umbilical cord. Finally, its 

 contents are completely absorbed and the sac disappears, 

 the ectoderm merging in the ectoderm of the body at the 

 point of attachment, and the endoderm in the intestinal wall. 



At an early period of the embryonic life the gills are pro- 

 duced into long filaments or external gills, the hyoid and the 

 other gills contributing to form a large number of long 

 filamentous external gills. They remain until the time of 

 hatching approaches, when they are gradually atrophied. 



