IX 



MOLLUSCA 



355 



the already existing blastomeres undergo division, and in this way 

 a one-layered sheet of cells, or " blastoderm," becomes spread over 

 the upper surface of the egg. 



When the blastoderm has extended so as to form a sheet of cells 

 covering about one-eighth of the surface of the egg, the formation of 

 ne,w blastomeres from the blastocones ceases : these latter now appear 

 as a series of narrow, spoke-like pillars radiating from the blastoderm 

 as a hub. The basal portions of these spokes become narrower and 

 narrower, as their nuclei wander farther and farther away from the 



y 



K- 



FIG. 285. Segmenting egg (32-cell stage) of Scpiii nj/fi-iiin/ >'.> viewed from the animal pole. 

 (Alter Koeppern, from Minchin's preparations.) 



')/<, blasfcoconi- ; '<//, blastomere. 



1 tlustoderin, till they become mere threads. Finally the portion with the 

 nucleus separates altogether from the blastoderm and begins to divide, 

 and in this way a membrane is formed consisting of a single layer 

 of extremely flattened cells, which rapidly extends all over the 

 yolk ; it extends also underneath the blastoderm so as to cover the 

 yolk there also. This sheet of cells is termed the yolk-membrane ; 

 it has been rightly compared to a portion of the endoderm. Its cells 

 exercise some digestive influence on the yolk and gradually change 

 it into a form that can be used for the nourishment of the embryo. 

 But no part of this yolk-membrane is incorporated in the permanent 

 epithelium of the mid - gut. The cells from which the mid-gut 

 epithelium will be formed, arise later by divisions of the most 



