178 



EDWIN G. CONKLIN 



TABLE 5 

 Cell size and cell number in the development of Crepidula and Fulgur 



SPECIES 



C. plana 



C. fornicata. . 

 C. convexa. . , 

 C. adunca. . . 

 Fulgur carica 



This table shows that although there are many more ectoderm 

 cells in the larger eggs of Crepidula than in the smaller ones, this 

 increased number is chiefly confined to the oral pole, where the 

 primordia of the various organs are located, and it also indicates, 

 as was emphasized above, that the increase in the number of cells 

 in C. adunca as compared with C. plana, is due to a greater num- 

 ber of non-differential divisions in the former species after the 

 primordia have been estabUshed. 



It seems probable that the more frequent divisions of the ecto- 

 meres in C. adunca as compared with those of C. plana is associ- 

 ated with their larger initial size, but at present it is not possible 

 to determine why the smaller cells of the latter species continue 

 to grow and divide for a longer period than the larger cells of the 

 former. 



e. Larvae. Finally the body size and cell size of fully formed lar- 

 vae of C. plana, C. convexa, and C. adunca are given in table 6. 

 In the case of C. plana the larvae measured were of maximum size 

 and were ready to escape from the egg capsules; in the other 

 species the larvae were of a corresponding stage of differentiation, 

 with velum and larval organs of maximum size, though these 

 larvae undergo metamorphosis within the egg capsules and do 

 not escape until they are adult in form. The body volume of 

 these larvae was roughly determined by measuring the length and 

 breadth of the body as seen from the dorsal side, the thickness of 

 the body being approximately the same as its width. 



