20 The Unity of the Organism 



of the question, namely that we have to do on the one hand 

 with attributes of the egg itself which are results or termini 

 of the kind that gave rise to the conception of heredity ; 

 and on the other hand with these same attributes not as 

 results but as causes or forerunners of later-appearing at- 

 tributes, also by the same criteria due to heredity. That the 

 egg attributes with which we are dealing are due to heredity 

 is obvious from the fortunate circumstance that, as already 

 indicated, the observations under review were comparative 

 to considerable extent. Thus we have this general com- 

 parison of the unripe eggs of the three species : "In the liv- 

 ing eggs of Cynthia this peripheral layer is clear and trans- 

 parent and contains uniformly but sparsely distributed 

 yellow pigment, which seems to be associated with these small 

 refractive spherules. ... In Ciona and Molgula also these 

 three areas are distinguishable in the living ^gg before ma- 

 turation, but not so clearly as in Cynthia. In Ciona the 

 peripheral layer is nearly transparent, the j^olk is a brown- 

 ish red. ... In Molgula both the peripheral layer and the 

 germinal vesicle are transparent, while the yolk is gray, 

 with a faint lilac tinge." ^ As an example of a very definite 

 statement of specific difference between the eggs we read, 

 "In Ciona the same type of protoplasmic movement occurs 

 as in Cynthia, but with certain minor differences." ^ 



The evidence brought forward by Conklin of species differ- 

 ences in eggs is not restricted by any means to Ascidians. 

 In some of the gastropod mollusca, for example, similar re- 

 sults were reached by a very notable research on the quanti- 

 tative relations of various Qgg organs of several species of 

 the genus Crepidula. To be explicit in a single case only, 

 the relative volume of macromeres 8A-3D to 3a-3d was 

 found to be 12.1:1 in Crepidula plana and 59.3:1 in C. 

 convex. From a long series of determinations of difference, 

 Conklin remarks : "One cannot study the eggs of different 

 animals without being much impressed with the fact that 



