THE DEVELOPMENT OF A MOLLUSK 31 



forms has become known, comparisons such as these have thrown 

 much Hght on the connections existing between various classes 

 of animals, the extent to which developmental histories corre- 

 spond being, in a degree, an index of relationships. 



With a view primarily to increase the embryological evidences 

 of evolution and at the same time to gain a clearer conception of 

 relationships, the development of all the various types began to 

 be traced from the original germ layers. Natiirally the condit- 

 ions which might influence development were considered, and ex- 

 planations of how the mechanical action of simple physical 

 factors, such as pressure, cohesion and gravity, might tend to 

 cause a dividing egg of a given character to assume successively 

 the various forms through w^hich it passes during development, 

 were soon advanced and received with great enthusiasm. To 

 determine exactly how important a role these extrinsic factors 

 play, and the extent to which the future form of an organism is 

 predetermined by the intrinsic character of the egg is evidently 

 of the greatest importance in the solution of the problem of 

 heredity and constitutes at present one of the main problems of 

 embryology . 



Although the earlier embryologists were satisfied with 

 simply tracing the origin of the various organs of the body 

 from their primary germ layers which begin to be defined 

 with the gastrula stage, nowadays the solution of the origin of 

 every organ or feature of the body and the significance and 

 factors of every step in development are sought by the most 

 painstaking tracing of the history of every single cell arising by 

 every succeeding division of the egg. It was with a purpose such 

 as this that an elaborate and careful study of the development of 

 Crepidida was undertaken by Prof. E. G. Conklin, of the Univer- 

 sity of Pennsylvania. This study has been followed by the 

 author in constructing for the American Museum of Natural 

 History the series of models described in the present paper. 



[7] 



