BODY SIZE AND CELL SIZE 



1^79 



TABLE 6 

 Body size and cell size of fully formed larvae of Crepidula 



Dorsal view 



Dimensions of body 



Length 



Breadth 



Relative vohimes 



Dimensions of cells 



Oesophagus ! 6 x 12 



Stomach (pyloris) 



Foot epithelium 



Gill epithelium 



Retina (post. wall)-. 



Sex cells (?) 



While the relative volumes of the unsegmented eggs of these 

 three species are as 1 : 7 : 24, the relative volumes of the larvae are 

 as 1 : 3 : 10; in short, the growth of the embryo and larva of C. 

 plana has been more rapid than that of C. convexa and of C. 

 adunca, so that the great disproportion which existed between the 

 eggs of these species at the beginning of development, has begun 

 to disappear. 



Likewise the cell dimensions of the larvae of these three species 

 show that the great disproportion in size of the early cleavage cells 

 of C. plana, as compared with C. convexa or C. adunca, has begun 

 to disappear. The cells of the larvae of C. plana are but little 

 smaller than those of C. convexa and C. adunca, but they are 

 much fewer in number. Even among the larvae differences in 

 body size are due chiefly to differences in cell number, rather than 

 to differences in cell size, just as is true of adults. 



Evidently growth in volume during embryonic stages has been 

 more rapid in C. plana than in the other species named. This 

 may be due to a greater absorption of water on the part of the 

 embryo of C. plana; and in accordance with this suggestion it 

 may be said that the cytoplasm of this embryo is less dense 

 and stains more faintly than that of the embryos of the other 

 species. Davenport ('97) showed, long ago, that the increase in 

 bulk of the frog embryo and larva, up to the time when it begins 



