16 The Unity of the Organism 



with the sperm. Nevertheless we are bound to recognize 

 that the egg no less than the sperm has hereditary attri- 

 butes of its own, and that other substances than chromatin 

 play a demonstrable part in the production of these. In- 

 deed the main discoveries concerning what in an earlier 

 chapter was called the promorphology of the egg are of this 

 sort. There is one kind of promorphology that is of special 

 importance to the present stage of this discussion. I refer 

 to the kind known sometimes as "germinal localization" and 

 sometimes as "organ forming substances" in the ovum. The 

 idea, expressed in a sentence, is that in the eggs of some 

 animals, portions of the egg destined to give rise to par- 

 ticular parts of the future embryo are visibly different from 

 other portions before cell division begins, in some cases even 

 before maturation and fertilization occur. According to 

 our understanding of heredity, these distinguishable por- 

 tions of such eggs are themselves hereditary attributes not 

 only of the animal species to which the eggs belong, but of 

 the eggs, no less than are distinctive morphological features 

 of the adult animals or of any developmental stages. The 

 study of these attributes of eggs is peculiarly interesting 

 since, belonging to germ-cells par excellence, if we can get 

 observational evidence on both their origin and destination, 

 we shall have direct evidence that one and the same substance 

 is determined on the one hand by heredity, and on the other 

 is a determiner in the strict genetic sense of hereditary at- 

 tributes yet to be developed. 



(a) Eggs of Ascidians--The Facts 



Because of the great importance of the observations of 

 E. G. Conklin in this field, and of his general views con- 

 cerning the bearing of his observations on heredity, we shall 

 make his work the center of our examination. One of the 

 most important of Conklin's investigations was on the eggs 



