1 44 CONK LIN. [Vol. X 1 1 1 . 



sometimes finds one of these masses lying near an embryo, 

 but wholly free from it ; I conclude, therefore, that they are 

 thrown away. 



When the embryos are stained with haematoxylin, the gran- 

 ular contents of the excretory cells stain dark carmine, while 

 all the remainder of the embryo stains a royal purple, or dark 

 blue. This carmine color, as is well known, can be produced 

 by treating haematoxylin with weak acids, and the fact that 

 these excretory cells stain a carmine color may indicate that 

 they contain some acid secretion. 



Heymons has traced with great care the history of the 

 external excretory cells in Umbrella. They are derived from 

 the cells 3c'' and 3d''. These cells divide, and then sink into 

 the interior of the body and are overgrown by ectoderm cells. 

 They are afterwards filled with brown concrement particles. 

 Generally the right excretory cells develop, while the left do 

 not. 



These cells lie near the anal cells, at the posterior end of the 

 embryo, and far removed from the velum and foot. I cannot, 

 therefore, believe that they correspond in cell origin to the 

 excretory cells of Crepidula, although 3c'' and 3d'' in Cre- 

 pidula repeat in a most remarkable way all the peculiarities of 

 the same cells in Umbrella, having been formed by bilateral 

 cleavage, and being large, clear cells with vesicular nuclei. I 

 have not been able to trace the cell origin of the excretory 

 cells in Crepidula, and the possibility remains that they are 

 derived from 3c'-' and 3d'-', as in Umbrella. If this be the 

 case these cells must be displaced much farther forward than 

 in Umbrella, possibly by the more active division of the ecto- 

 derm cells near the growing-point. But on the other hand it 

 is possible, as is evidenced by the dissimilarity in their later 

 history, that the excretory cells in Umbrella and Crepidula 

 are not homologous, and that they do not have the same cell 

 origin. 



Heymons does not consider the differences between these 

 external excretory cells of prosobranchs and opisthobranchs to 

 be any serious objection to their homology. He says (p. 293): 

 " Der Umstand, dass die ausseren Urnieren der Prosobranchier 



