been described, or are familiar to every worker on lamellibranch 

 embryology. 



It will accordingly be necessary to describe the formation of 

 the embryos only very briefly. As the age of the eggs could not 

 be accurately determined it is not possible to give the exact time 

 that elapsed before the polar bodies made their appearance, but 

 probably the first polar body was given off in from half to three- 

 quarters of an hour after the egg was laid. The first external 

 sign of activity after fertilization is the formation of a promi- 

 nent yolk-lobe, which nearly disappears after the first polar body 

 is formed, to become prominent again when the second polar 

 body is formed (fig. 27) and to disappear again after this is 

 separated from the egg. It again becomes prominent when the 

 egg cleaves into two cells, (fig. 29) and is slightly visible during 

 the second cleavage. The polar bodies are given off from the side 

 of the egg that is opposite the yolk-lobe, and although the egg 

 is not inclosed in a membrane as is the case in many forms, the 

 polar bodies adhere until the cells are provided with cilia and 

 the embryo begins to swim. The adherence is apparently due 

 to protoplasmic strands such as have been described by Andrews 

 (i and 2). 



The first plane of cleavage passes through the point where 

 the polar bodies were formed, and just to one side of the yolk- 

 lobe (fig. 29). This divides the egg into two unequal portions, 

 the larger of which contains the whole of the yolk-lobe. The 

 next cleavage plane also passes through the point where the 

 polar bodies were formed and nearly at right angles to the first 

 cleavage plane (fig. 30). This also passes a little to one side 

 of the yolk-lobe so at least a large portion of the yolk remains 

 in one cell which is larger than the others. 



The division into eight cells is accomplished by cleavage 

 planes at right angles to the planes already described (fig. 31). 

 In this way each of the four cells are divided unequally, those 

 nearest the polar bodies being smaller than those on the opposite 

 side. Continued division of the cells results in the formation 

 of a mass of cells (fig. 32) some of which are confined to the 

 surface, while others are large and extend into the interior, thus 

 forming an almost typical epibolic gastrula. This stage of 

 development is reached in from 12 to 14 hours, at which time 



55 



