No. I.] THE EMBRYOLOGY OF CREPIDULA. i6l 



comes to inquire into the history of individual blastomeres, there 

 is in general the same lack of data which was noticed in the 

 study of the entoblast and mesoblast. In this case, therefore, 

 as in previous instances, I am compelled to limit comparisons 

 almost entirely to the work of Blochmann and Heymons. 



I am wholly unable to bring Blochmann's account of the 

 history and fate of the entoderm cells into accord with the 

 work of either Heymons or myself. His figures would indi- 

 cate that throughout the whole cleavage there is a large seg- 

 mentation cavity between the four macromeres (see especially 

 his Figs. 40, 49, 51, 58, 60, 64, 6^, 71). Three small cells 

 (en^,, en^, en^) which apparently correspond to 4A, 4B, and 4C 

 in Crepidula are first shown at the vegetal pole. Figs. 54, 55. 

 Afterwards these cells pass up throtigh the segmentation cavity, 

 Figs. 63, 64, until they come to lie on the upper side of the 

 macromeres. In the meantime two new cells are formed (eng^, 

 en^'), which have no homologues elsewhere so far as I know, 

 and these cells also pass up through the segmentation cavity 

 to the upper side of the macromeres. These small entoderm 

 cells then divide, forming a cap of small protoplasmic cells 

 which lie above the yolk cells and form the roof of the archen- 

 teron. The latter cavity is in communication with the exterior 

 throtigh the long and narrow segmentation cavity into which the 

 blastopore opens on the ventral side. These small cells later 

 form the intestine. 



This process is so absolutely unique that Blochmann's account 

 may well be doubted. Inasmuch as the macromeres or their 

 derivatives, which are located at the middle of the ventral side, 

 are in many cases invaginated, I once thought it possible that 

 these small cells might correspond to the macromeres, A, B, 

 C, D, while the large yolk cells might represent the cells of 

 the fourth or fifth quartettes ; however, this view cannot be 

 upheld, for these four large yolk cells can be followed right 

 through the development, and are undoubtedly the macro- 

 meres A, B, C, D. At present it is altogether useless to 

 attempt a detailed comparison of the entoderm cells of Neri- 

 tina with those of other gasteropods. 



In Umbrella the macromere D is smaller than the other 



