No. I.] THE EMBRYOLOGY OF CREPIDULA. 65 



the interpretation of later stages, it demands a careful consid- 

 eration. 



Heymons ('93) called attention to the difference between 

 Neritina and Umbrella in this cleavage, and he ascribed it to 

 the difference in the axial relations of the "cross" of ecto- 

 blast cells in those two animals. In this he was certainly in 

 error, as we shall see when we come to consider the cross in a 

 subsequent section. 



More recently Kofoid ('94) has discussed this unusual cleav- 

 age in Neritina and has presented strong evidence for believ- 

 ing that Blochmann was mistaken in his interpretation of it, 

 and still more recently Lillie ('95) cites Kofoid' s criticism with 

 approval. Kofoid suggests a possible correction of Bloch- 

 mann's interpretation (the nature of which is shown in the 

 accompanying diagram, 5c), which would bring the cleavage 

 of Neritina into conformity with the "law of alternating 

 cleavages," but curiously enough, he just misses the true ex- 

 planation. The modification suggested by Kofoid does meet 

 the requirements of his law of alternating cleavages, but it does 

 not harmonize with Blochmann' s oft-repeated statement that 

 the terminal cells in the transverse arms of the cross (his 

 " Urvelarzellen " ) come from two cells, 2a and 2c, of the 

 second quartette. Even before cleavage begins two masses 

 of granules can be recognized on opposite sides of the animal 

 pole, and in the formation of the second quartette these 

 granules pass into the cells 2a and 2C, and finally they appear 

 in the " Urvelarzellen," as soon as these are formed. Owing 

 to the presence of these peculiar granules, it seems very improb- 

 able that Blochmann could have been mistaken in the deriva- 

 tion of the "Urvelarzellen." Kofoid recognizes this difficulty 

 and attempts to meet it by suggesting that the granules 

 originally present in the cells 2a and 2c may disappear, and 

 that new granules may appear in the corresponding cells of 

 the third quartette (3a and 3 c), which are in turn handed over 

 to the "Urvelarzellen." This suggestion seems to me as im- 

 probable as it is unnecessary. In Crepidula the terminal cells 

 of the cross (" Urvelarzellen ") are derived exactly as Bloch- 

 mann asserts is the case in Neritina, and the same thing is 



