Evidence from Metazoan Germ-Cells 21 



the distribution of yolk to the four macromeres is highly 

 characteristic of different species and orders." Yet, as 

 we shall have occasion to point out later, it seems certain 

 that facts of this kind have deeper meanings than Conklin 

 has appreciated. 



After all that has been said in previous pages about 

 specific differences, the point aimed at here will be obvious. 

 The "decidedly thicker" layer of peripheral protoplasm at 

 the lower pole in Ciona than in Styela, and the brownish 

 red yolk in Ciona as contrasted with the white yolk tinged 

 with lilac in Molgula, are indeed "minor differences." But 

 species differences in adults and in all other stages are minor 

 as a rule. The difference between the transparent-white 

 color of the full-grown Ciona intestmalis and the pale, green- 

 ish yellow of the adult Molgula manhattensis is also minor 

 as contrasted with the "same type" of organization of the 

 two animals. But it is just this being true to type as re- 

 gards minor differences in living beings that has given rise 

 to the conception not only of organic species, but to a large 

 extent, of heredity as well; and how can any one recognize 

 the differential color of the grown-up Ciona as compared 

 with the grown-up Molgula as due to heredity, but refuse 

 to recognize the differential color of the eggs of the two 

 animals as due to the same cause? 



We are, then, bound to accept the attributes of the eggs 

 as the results of heredity, and so inquire about the "physi- 

 cal basis" or "bearers" of these attributes. And this brings 

 us again to the main issue of this section: Have we or have 

 we not observational evidence that any other substance or 

 substances than chromatin contribute to the production of 

 the egg-phenomena under consideration? 



Conklin's conclusions and mode of reasoning as to the 

 bearings of his observations on heredity are so crucial for 

 the point now engaging us that we must examine them in 

 considerable detail. Returning to the summary of results 



