Heredity in Fundulus Hybrids 157 



nucleus and cytoplasm; but the fact remains that at the time of 

 fertilization the hereditary potencies of the two germ cells are not 

 equal, all the early development, including the polarity, symmetry, 

 type of cleavage, and the relative positions and proportions of the 

 future organs being predetermined 'in the cytoplasm of egg cell, while 

 only the differentiations of later development are influenced by the 

 sperm. In short, the egg cytoplasm fixes the type of development and 

 the sperm and egg nuclei supply only the details. " 



My own observations on the early stages of the process of 

 heredity and an examination of the experimental evidence, that 

 lies at the foundation of the above view, together with a number ot 

 more recent contributions along the same line, force me to take a 

 position on certain questions decidedly opposed to that of Conk- 

 lin. 



Is the specific symmerty, polarity, etc., expressed solely in the egg 

 and not in the spermatozoon or in the various types of somatic cells? 

 It is scarcely necessary to point out that the sperm cell at all 

 stages of development shows just as pronounced a polarity as the 

 egg — more so in later stages. This polarity is largely the expres- 

 sion of a definite relationship between nucleus and cytoplasm and 

 is doubtless specific and hence characteristic of all cells of a given 

 organism. No doubt this polarity expresses itself in a somewhat 

 different fashion in different kinds of cells, but these special mani- 

 festations are, I believe, of secondary importance. When the 

 spermatozoon for example, undergoes an exaggeration of its 

 specific polarity and symmetry during the end stages of its devel- 

 opment, when most of its cytoplasm is converted into a locomo- 

 tor mechanism, it becomes a specialized cell with a definite func- 

 tion, and departs from the specific cell type. Is not the same 

 true of the egg, a cell in which the primitive specific polarity and 

 symmetry have been distorted by the large accumulations of inert 

 nutritive material? It is entirely probable therefore, as LiUie has 

 shown, that the real polarity and symmetry are characters of the 

 ground substance common to all of the cells of the organism. 



Since then a fertilized egg is a product of the more or less com- 

 plete fusion of two cells with the same inherent specific polarity 

 and symmetry, it appears somewhat extreme to state that "all 



