328 PROCEEDINGS OF THE ACADEMY OF [Julv, 



a considerable quantity of 3'olk. In all of these regards it resembles 

 the corresponding cell in all annelids and mollusks, with the exception 

 of the cephalopods, in which the cell-lineage has been studied. 



But the feature of particular interest in this cell is its destin}^ rather 

 than its origin. In all annelids and mollusks, so far as known, the 

 anterior portion of this cell gives rise to most of the mesoderm, while 

 from its posterior portion the terminal part of the alimentary canal 

 arises. This cell is therefore known as the mesentoblast. The history 

 of this cell in Fulgur conforms entirely to what we know of it in other 

 forms, as will appear later. 



With the formation of the mesentoblast the segregation of the sub- 

 stances of the different germinal layers is nearly completed. I have 

 not observed that the sul^stances composing the mesentoblast differ 

 visibly from those of the ectoblasts and entoblasts, except in the 

 quantity of j^olk which these different cells contain. But in Limncca, 

 Physa and Planorhis I have been able to distinguish the substance of 

 the ectomeres from that of the mesomeres and entomeres before the 

 first cleavage of the egg. In these fresh-water snails, therefore, there 

 is a definite localization of the substances of the different germinal 

 layers, and I have elsewhere shown reason for believing that it is the 

 fact of this localization which determines that the ectoblast shall be 

 separated in three quartets, and that the mesoblast shall arise from 

 one cell of the fourth c[uartet. Since the segregation of the germinal 

 layers occurs in exactly the same Avay in Fulgur as in these other 

 gasteropods, it seems probable that there is here also the same type 

 of localization of the germinal substances of the egg, and that the enor- 

 mous accumulation of yolk in this egg has not altered in any funda- 

 mental way the localization of its various substances. 



The other members of the fourth quartet, 4A, 45 and 4C, are not 

 formed until the 50-cell stage (fig. 9). They are much larger than the 

 cell 4(i and contain a much greater quantity of yolk. Because of their 

 great size I sliall speak of them as the "secondary macromeres." 

 These cells are purely entoblastic in character and give rise to a por- 

 tion of the alimentary canal. In every one of these details they 

 closely resemble the corresponding cells of Crepidula. 



4. FURTHER HISTORY OF THE ECTOMERES. 



The further history of the three c^uartets of ectomeres may now be 

 followed through the development up to the time when they give rise 

 to the first recognizable organs. About the time of the formation of 

 the cell M all of the twenty micromeres divide so that forty micro- 



