72 



CONKLIN. 



[Vol. XIII. 



briefly indicated by the following tabular comparison of the lin- 

 eage of 4d in these two animals. To facilitate comparison I use 

 throughout the same designations for Crepidula and Umbrella. 



CELL STAGE 25 



-hi 



46 60 



y. 





MM (right)^MM'<^ 

 MM'(L£Ft)^ 



m 



m— 



1/M 

 M- 



^m /r 



■M' 



-M' 

 • m' 



M' 



CREPIDULA D<^ 



/M/^'(RiGHT]<;] 

 MA\'(L£frj<; 



<x 





.< 



< 



^-Mi^. 





m 



Cell Stage ^5 



50 



44 



49 



68 



7-7 



Heymons' figures indicate that the resemblance is much 

 closer than the above-given table would show, e.g., all the cells 

 designated by capitals are large cells containing a considerable 

 quantity of yolk ; they are about equal in size, are grouped in 

 a characteristic way, and in every respect resemble the entero- 

 blasts of Crepidula. Again, the second division of 4d is equal 

 in Crepidula, unequal in Umbrella ; the third division is un- 

 equal in Crepidula, equal in Umbrella. The time relations of 

 these two divisions are simply reversed. 



Although the formation and subsequent divisions of the cell 

 4d are so similar in Crepidula and Umbrella, my interpretation 

 of the destiny of some of the cells derived from this blastomere 

 is wholly different from that given by Heymons. According 

 to this author, the entire cell 4d and all of its derivatives are 

 purely mesoblastic. Heymons has followed all of the divisions 



