No. I.] THE EMBRYOLOGY OF CREPIDULA. 5 



more about these normal processes before we can properly 

 understand abnormal ones. In order to know the significance 

 of cleavage, it is necessary not only to find out how much the 

 egg may be fragmented or the blastomeres transposed without 

 irreparably destroying development, but also and much more, 

 it is necessary to know every step in the normal formation of 

 the embryo. It is less important to know what remedial 

 processes Nature may have for healing broken eggs, than to 

 understand her usual methods of developing unbroken ones. 

 Whether and how much this " secondary," or regenerative 

 development may differ from the "primary," or normal, is still 

 an open question. If there be a difference, as Roux ('93) 

 maintains, the phenomena of regenerative or secondary develop- 

 ment are much more complicated and difficult of explanation 

 than the process of primary or normal development, since in 

 these cases we have to explain the phenomena of normal devel- 

 opment plus those of regeneration. In any case the phenomena 

 of normal development are the ones to be explained, whatever 

 method may be used ; and before any explanation can be given 

 it is necessary to know the usual development as thoroughly 

 as possible. 



It is because of the perennial interest in these questions of 

 the earliest homologies, and of the significance and causes of 

 the various forms of cleavage, and also with the hope that I 

 may be able either directly or indirectly to add something, 

 however little, to the solution of some of these problems, that 

 I now bring forward this long-delayed contribution on the 

 Embryology of Crepidula. 



Crepidula is a genus of prosobranchiate gasteropods, whose 

 development has never heretofore been studied so far as I can 

 learn, — a genus, moreover, which is in many respects a very 

 interesting one, apart from its embryology ; besides, it is so 

 abundant all along our Atlantic coast from Labrador to Florida, 

 and its eggs are so easily obtained, so numerous, and so exceed- 

 ingly favorable for embryological research, that it seems 

 remarkable that no one has hitherto attempted to study its 

 development. 



