No. I.] THE EMBRYOLOGY OF CREPIDULA. 25 



laid is, in general, directly proportional to the volume of the 

 adult. This is very plainly the case within a single species 

 where the number of eggs laid always stands in direct relation 

 to the size of the animal which lays them. When one species 

 is compared with another this same thing is generally true of 

 the total volmne of eggs laid, i.e., the species with the largest 

 individuals lays the largest volume of eggs, though the number 

 and size of the eggs in the various species differs immensely. 

 For example, C. convexa, which is about one-twentieth the 

 size of C. fornicata, produces only about one-sixtieth as many 

 eggs of one-fifteenth the total volume of those laid by the 

 latter species. The great reduction in the number and total 

 volume of eggs in C. convexa and C. adunca, as compared with 

 the other species, is made possible by their foetal type of 

 development. At the same time the wide distribution of 

 individuals brought about by the free-swimming veligers of C. 

 fornicata and C. plana is partially secured in C. convexa (I do 

 not know whether this is true of C. adunca or not) by the 

 freely moving young or spat of this species, which are much 

 more active than the spat of either of the other species. 



It is very evident that the foetal type of development in C. 

 convexa and C. adunca is correlated with the smaller size of 

 the adult in these species, and for the reasons given above, it 

 seems to me probable that the former may be in some way the 

 result of the latter.^ 



4. General Sketch of the Embryology. 



The First and Second Cleavages. — The chief axis of the ovum 

 corresponds to the future dorso-ventral axis of the embryo. 



1 Although I do not suppose that these relations between the size of the adult 

 and the number, size, and volume of the eggs produced, is a general law applicable 

 to all larval and foetal types of development, neither do I think that such relations 

 are wholly isolated, i.e., true only of this one genus, Crepidula. I believe they 

 will be found to be quite generally true of the gasteropods. Long ago, Fol ('76) 

 called attention to the fact that among the heteropods the smallest species lay 

 the largest eggs. He says, " The smallest heteropods lay relatively the largest 

 eggs, but infinitely fewer than the larger species." He did not observe that the 

 largest eggs had a foetal or suppressed larval development, but I think it would 

 be safe to assume that this is true, and that here also the foetal type, and conse- 

 quently the larger eggs, are due in part to the smaller size of the adult. 



