The Development of IschnochitOD. 615 



Comparisons. 



In all other forms whose development is accurately known the 

 third quartette is very much smaller than in Ischnochiton and the 

 part it performs in the development is not so striking. Whether or 

 not it is as important cannot be answered at the present time since 

 no one has described the cleavages beyond the earlier stages. 



In Umbrella and Crepidula among Molluscs the first division of 

 this quartette is nearly radial, but a shifting occurs which throws the 

 cells into the same position as in Ischnochiton. The next division 

 affects the upper cells cleaving them bilaterally , while a subsequent 

 cleavage divides the lower cell (stomatoblast) into two equal halves 

 (this is not described in the two posterior quadrants in Umbrella). 

 These last two divisions are very similar in Crepidula and Umbrella'- 

 and correspond closely to the divisions of this quartette in Ischnochiton. 

 Beyond this stage no cleavages in the anterior quadrants are figured 

 in Umbrella, and in the posterior modifications arise owing possibly 

 to the fact that some of these cells become excretory. 



In Crepidula the outer cells, corresponding to the posterior third 

 quartette stomatoblasts 3c^-^ and 3d^-^ in Ischnochiton, divide into 

 upper and lower products. This cleavage occurs in this direction in 

 Chiton but much later in the development. Also there is another 

 relatively accelerated cleavage in Crepidula in the division corresponding 

 to 3c^-^ and 3d^-^. The corresponding cleavage in the anterior 

 quadrants occurs much later, at which time both upper cells 5«^.^, 

 5 a ^.-2 and 3 b ^'^, 3h^-'^ divide. Hence in the anterior quadrants 

 the two third quartette groups (Conklin's fig. 47) at this time bear 

 a striking resemblance to our Fig. 30. There are two lower cells in 

 each group corresponding to the stomatoblasts, two median cells and 

 two upper that have the same origin and position in the two forms. 



Regarding the later history of these cells nothing is accurately 

 known. 



VIII. All Account of the Invagination witli Comparisons. 



After the formation of the third quartette, the macromeres lying 

 at the vegetative pole occupy the furrow between the members of this 

 quartette. A segmentation cavity of considerable size is now present 

 but they project into it but little. With the formation of the stomato- 

 blasts of the third quartette, however, the ectoderm extends nearer 

 the vegetative pole and in proportion as the macromeres are encroached 



Zool. Jahrb. XII. Abth. f. Morph. /(.Q 



