62 CONK LIN. [Vol. XIII. 



continue to be budded off from the macromeres for a much 

 longer period than in some other forms ; thus, for instance, 

 Blochmann describes only three generations of spherules from 

 the macromeres in Neritina. In Nassa, according to Bobretzky, 

 a greater number are formed ; he describes twenty spherules 

 as arising in this manner, and it is possible that more arise in 

 the same way in later stages. Probably the amount of yolk 

 present influences the number of spherules so formed ; in other 

 words, the greater the number of spherules required to sur- 

 round the macromeres, the more frequently are generations 

 formed from the macromeres." Lillie ('95) comments upon 

 this passage, and justly remarks that "the important point is to 

 determine how many of these generations are ectomeres," not 

 all micromeres being ectomeres, as Wilson ('92) has shown in 

 the case of Polymnia and Aricia. 



In all four species of the genus Crepidula which I have 

 studied, but three quartettes of micromeres are separated from 

 the macromeres, and this in spite of the fact that the Q.gg of 

 C. adunca is twenty-seven times as large as the ^gg of C. 

 plana ; in this case, therefore, the size of the &gg has no 

 influence on the number of quartettes separated from the 

 macromeres, although a count of the nuclei shows that about 

 five times as many ectoderm cells are present in C. adunca at 

 the time of the closure of the blastopore as are found in C. 

 plana. This increased number of ectoderm cells in the large 

 egg is due entirely to the more rapid division of the three 

 quartettes already formed, and not to the formation of addi- 

 tional quartettes. 



McMurrich's conclusions are so much in conflict with my 

 observations on Crepidula that I have taken the pains to briefly 

 study the cleavage of Fulgur. The result of this study shows 

 that in this case also three, and only three, quartettes of micro- 

 meres are separated from the macromeres. The cleavage is 

 marvellously like that of Crepidula, though the eggs are from 

 fifty to one hundred and forty times as large. 



Two other somewhat doubtful exceptions to this rule have 

 been recorded; e.g., Salensky ('87) believed that more than three 

 quartettes of micromeres were formed in Vermetus. More 



