EMBRYOLOGY 



487 



burrows its way between the muscle 

 notochord and forms from its wall the 

 sheath of the notochord and nerve cord, 

 is known as the sclerotome, 

 the part which forms the 

 muscles being known as 

 the myotome. Lastly, a 

 downgrowth from each 

 dorsal division in the 

 pharyngeal region gives 

 rise to a gonad. 



During the external and 

 internal changes B 



Habits of the which we haye 



traced, the larval 

 Ampkioxus swims freely 

 in the sea, usually at a 

 depth of a few fathoms 

 from the surface. As its c 

 metamorphosis reaches 

 completion, it sinks to the 

 bottom and takes up the 

 burrowing habits of the 

 adult. 



The ovum of the frog has 

 been described 



Segmentation. on P- ?*• . The 



first division of 

 its cleavage (p. 109) forms 

 two similar cells, each con- 

 taining, like the ovum, an 

 upper, black, pigmented E 

 portion and a lower, 



Fig. 366. — Stages in the cleav- 

 age of the frog's egg. Of each 

 stage two views are given, 

 one showing it from the side, 

 the other obliquely from 

 above. A-F show succes- 

 sively stages with two, four, 

 eight, sixteen, thirty-two, and 

 numerous blastomeres, 



plate and the 



connective-tissue 



This outgrowth 



