56 



S. HATTA. THE FATE OF THE PERISTOMAL 



largest ventral part of which is in early stages nothing more than a 

 spherical mass of the yolk-loaded cells, is very slow to be differentiated. 



This mass of yolk- 



cells in development 

 lays much behind the 

 above mentioned 

 dorsal organs (such 

 as the mesoderm, the 

 chorda, and the me- 

 dullary cord), so that 

 this part of the em 

 bryo when seen from 

 the side is later almost 

 completely surround- 

 ed by the narrow 

 dorsal zone formed of 

 the latter group of or- 

 gans and so the anal 

 opening is brought 

 under the crooked 

 head protuberance, 

 so as to be concealed by the latter. This peculiar feature of the embryo 

 is seen in early part of the fourth day. In such an embryo as that 

 just mentioned, therefore, the body-axis describes a complete circle 

 (Photogram 2). The spherical belly of the embryo is by degrees reduced 

 in thickness, and the body is prolonged, so that the head is wound spirally 

 and is found at last resting either on the right or on the left side of the 

 body ; the body-axis describes now a spiral line. 



Subsequently the body is so prolonged as to be turned twice in spiral, 

 until on the thirteenth day the embryo breaks the chorion and is hatched 

 out. A just hatched embryo which lies flat on its right or left side on the 

 bottom of the vessel, is bent in a large curve, except its hind end which is 



Photogram 2. Sagittal section through the proximal part of 

 mesoderm of an embryo with 27 mesodermic somites. 

 ò. fore brain. /. pharynx, s. mesodermic somite. /. tail-bud. 



