No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



147 



Comparison of Later Stages in Nereis, Umbrella, and Crepidula, 



tettes at different stages for all forms with spiral cleavage 

 which have been sufficiently studied. It will be seen that up 

 to the 44-cell stage, beyond which it is impossible to carry the 

 comparison in most cases, the first quartette divides most fre- 

 quently in the annelid and the polyclade, the second and third 

 quartettes in the mollusk, and the fourth quartette in the poly- 

 clade and mollusk. A comparison of Nereis, Umbrella, and 

 Crepidula at the 58-cell stage shows still more plainly that the 

 divisions of the first quartette are very rapid in the annelid, 

 while those of the second and third quartettes are much more 

 rapid in the mollusk. 



While in general the number of cell divisions may be taken 

 as a measure of the development, precocity in cell division does 

 not always indicate precocity in differentiation ; e.g., in the case 

 of the first somatoblast (2d) the differentiation is much slower 

 in the gasteropod than in the annelid, and yet the cell divisions 

 are more numerous in the former than in the latter. Cell 

 division is not always associated with differentiation, and 

 therefore the measure of differentiation cannot always be 

 determined by the number of divisions. In the cases com- 

 pared above, however, there is no doubt that the more rapid 



