No. I.] THE EMBRYOLOGY OF CREPIDULA. 23 



Clearly two methods of reducing the total amount of germinal 

 matter are possible : {a) the germ cells, while remaining the 

 same in number, may decrease in size, or (b) the germ cells 

 may decrease in number, provided a larger proportion of them 

 produce adults. Both of these methods are illustrated within 

 the genus Crepidula. {ci) The typical form of C. plana, which 

 is about one-third the size of the average C. fornicata, pro- 

 duces almost as many eggs as the latter, but each egg is only 

 about one-third the size of the eggs of C. fornicata. In this 

 case the total amount of germinal matter has been decreased 

 (or increased, according as one or the other species is taken as 

 a standard) by the decrease in size of the individual cells. 

 (b) In the dwarf variety of C. plana, which is only one-thir- 

 teenth the size of the type form, the eggs are of the same size 

 as in the common variety, but much less mimeivus. The 

 method of development in the two varieties is exactly the 

 same, and therefore it follows that unless the typical variety 

 is rapidly increasing in numbers, which does not appear to be 

 true, the dwarf variety must be rapidly disappearing. I think 

 it altogether probable that the eggs laid by the dwarfs are not 

 numerous enough to continue the dwarf variety in its present 

 numbers, and it would rapidly disappear if it were a true or 

 morphological variety. However, since it is merely a physio- 

 logical variety of C. plana, due to the smaller size of the shell 

 in which the young take up their residence, the continuance 

 of the dwarf variety is not dependent upon the number of eggs 

 produced by the dwarfs ; rather it depends upon the number 

 of the young of C. plana, whether of the common or dwarfed 

 form, which make their abode in the smaller shells. 



In C. convexa and C. adunca the amount of germinal matter 

 is reduced in the same way that it is in the dwarfed form of 

 C. plana, i.e., by reducing the number of cells. Since, how- 

 ever, these are true species which are neither rapidly increas- 

 ing nor decreasing in numbers, it follows that if they produce 

 a smaller number of eggs than the other species, the chances 

 that each Qgg will produce an adult must be proportionately 

 increased. In C. convexa and C. adunca this is done simply 

 by lengthening the period during which the young organism 



