No. I.] 



THE EMBRYOLOGY OF CREPIDULA. 



21 



This dissimilarity in the size of 

 the eggs is due to the differences 

 in the larval history — the species 

 with the most pronounced larval 

 period having the smallest eggs, 

 because but a small amount of 

 nutritive yolk is necessary to carry 

 the development to the free- 

 swimming stage where the larva 

 can take care of itself, while in the 

 species without any larval history diagram i.- showing the relative size of 

 enough yolk must be stored in the *^ "^s" of c plana, c. fornicata, c. 



" ■' convexa, and C adunca. Ihe actual 



egg to carry the development clear diameter ana volume of each is given in 

 ,, ,, ,, 11, T,- millimeters and cubic millimeters. 



through to the adult condition. 



The larger size of the eggs in C. adunca and C. convexa as 

 compared with C. fornicata and C. plana, is due chiefly to the 

 greater amount of yolk stored in the entoderm cells of the two 

 former species ; and it is worthy of note that this increased 

 quantity of yolk is equally distributed, so that the four macro- 

 meres produced by the first two cleavages are nearly equal in 

 size and bear the same relation to each other in the larger 

 eggs that they do in the smaller ones, though in many other 

 molluscan eggs, e.g., Aplysia, Urosalpinx, Unio, and Ostrea, one 

 of the macromeres is very much larger than the other three. 



In spite of this vast difference in the size of the eggs in 

 these different species of Crepidula the cleavage, gastrulation, 

 and formation of organs is very similar in all of them. In the 

 large eggs of C. adunca and C. convexa the entoderm cells are, 

 relative to the ectoderm cells, much larger than in C. fornicata, 

 and C. plana ; therefore, at the time of the closure of the 

 blastopore there are more ectoderm cells in the large eggs than 

 in the small ones. A count of the nuclei of the ectoderm cells 

 in the species plana, fornicata, and convexa at this stage, shows 

 that they are to each other as two, three, and five respectively, 

 while a comparison of adunca with the other species at an 

 earlier stage (just before the division of the three smaller en- 

 toderm cells, see p. 1 56), shows that it has a larger number of 

 ectoderm cells than either of the others. 



