608 HAROLD HEATH, 



conflicts with the regular alternating cleavages and consequently with 

 Ischnochiton. However, very shortly after their formation in Crepi- 

 dula these blastomeres shift so that they have exactly the same 

 position in the two forms. 



The next two divisions (of 2a^-i and 2a^-i) are also similar, 

 but after this point comparisons cannot be drawn since at the next 

 divisions in Crepidula the cells corresponding to the post-trochal 

 cells divide a second time, cleavages which certainly do not occur in 

 Ischnochiton previous to the 300 or 400 cell stage. 



Therefore up to the time when there are seven cells in each 

 quadrant the same features of development obtain save in the one 

 case where the cleavage was slightly leiotropic. However, it must 

 not be understood that the second quartette in Ischnochiton v^hen 

 these cleavages are completed is similar to that of Crepidula, for the 

 cell 2a^-^ which in Crepidula has never been seen to divide in 

 Ischnochiton divides repeatedly. 



This brings out an interesting point in Crepidula, that while the 

 upper cells of this series are similarly arranged to those in Chiton 

 the division which produces the stomatoblast is delayed. This is 

 probably correlated with the late formation of the stomodaeum. 



Later stages in the cell lineage were not determined owing to a 

 lack of landmarks, which latter fact rendered it impossible to accu- 

 rately dififerentiate the second from the neighboring third quartettes. 

 However, in a general way these groups have been followed, and some 

 features in their development are strikingly like those in Ischnochiton. 

 For example, during the early stages of gastrulation the second 

 quartette cells extend from the arms of the cross to the angles of 

 the quadrangular blastopore, while the third quartette cells extending 

 the same distance alternate with them around the embryo. Comparing 

 this with Figs. 35, 39 in Ischnochiton we have exactly the same 

 conditions. Also it is believed that in all but the posterior quadrant 

 some cells from this quartette contribute to the formation of the 

 velum; in other words they form secondary trochoblasts as in 

 Ischnochiton. 



A growing point forms in the second quartette of the posterior 

 quadrant which also occurs in Chiton. 



Beyond this point comparisons cannot be made, but as far as it 

 has been traced the development in the two forms is very similar 

 and there is every reason to believe that if it were possible to carry 

 on the comparison into later stages these resemblances would not cease. 



