The Development of Ischnochiton. 607 



From this point I have been unable to trace the cell lineage of 

 the entire group. 



At a time shown in Fig. 41 the lower hemisphere is somewhat 

 triangular owing, as will appear later in the account, to a tendency 

 on the part of the third quartette cells to press in upon the arch- 

 enteron. At such a time the greater portion of the second quartette 

 stands out boldly, forming the angles of the triangle. Later however 

 when the embryo resumes a circular form the second quartette sinks 

 slightly below the general surface, forming grooves which are of the 

 greatest value in late stages as indicating the region of these groups 

 of cells. The anterior groove is wider and more open than the right 

 and left, and as the mouth shifts along this path I believe that many, 

 and possibly the majority, of the second quartette cells disappear into 

 the stomodaeum. There can scarcely be any doubt but that there is 

 an invagination of at least a part of the anterior second quartette up 

 to a time when the blastopore has shifted 45*^ and I believe it 

 continues until the shifting is complete. 



In the right and left groups of this quartette the invagination is 

 small as the mouth moves into its first position. The cells however 

 retain their connection with the blastopore and prototroch, and as the 

 shifting occurs they become curved until at the time when the mouth 

 has about reached its permanent position they are nearly perfect 

 semicircles bordering the third quartette cells lying on each side of 

 the mouth (Fig. 50 and c diagram E). 



We are now in a position to consider the fate of these three groups 

 of cells. As I have said I believe that little doubt exists that many 

 if not all the cells in the anterior quadrant become swallowed up 

 and enter the stomodaeum. The right and left groups are relatively 

 slender, forming narrow curved strips lateral to the mouth, and 

 probably contribute to the formation of the general surface of the body. 



Comparisons. 



CoNKLiN in his study of Crepidula has made exhaustive com- 

 parisons of various Molluscs relating to this quartette and finds very 

 close correspondence both as regards direction of cleavage and position 

 of cells. For this reason therefore I shall simply compare Crepidula 

 and Ischnochiton. 



The divisions up to the formation of 4 cells in each quadrant 

 are exactly as in Ischnochiton. But in the next cleavage in Crepi- 

 dula 2a^'-^ divides in a "slightly leiotropic" direction, and therein 



