Vi.] THEIR SIGNIFICANCE IN HEREDITY. 351 



that the process of fertilization is essentially the conjugation of 

 nuclei, even then there appeared to be no reason why the 

 number of divisions undergone by the nucleus of the mature 

 Qg'g should be looked upon as an essential feature. 



This was the state of the subject at the time when I first 

 made an attempt to ascertain the meaning of the formation 

 of polar bodies. I based my views upon the idea, which 

 was just then gaining ground, that Nageli's idioplasm was 

 to be sought for in the nucleus, and that the nucleoplasm must 

 therefore contain the substance which determines the form 

 and functions of the cell. Hence it followed that the germ- 

 plasm — the. substance which determines the course of embry- 

 onic development — must be identified with the nucleoplasm 

 of the egg-cell. The conception of germ-plasm was brought 

 forward by me before the appearance of Nageli's work ^ which 

 is so rich in fertile ideas ; and germ-plasm does not exactly 

 coincide with Nageli's idioplasm^ Germ-plasm is only a cer- 

 tain kind of idioplasm — viz. that contained in the germ-cell — ■ 

 and it is the most important of all idioplasms, because all the 

 other kinds are merely the results of the various ontogenetic 

 stages into which it developes. I attempted to show that the 

 molecular structure in these ontogenetic stages into which the 

 germ-plasm developes would become more and more unlike 

 that of the original structure of this substance, until it finally 

 attains a highly specialized character at the end of embryonic 

 development, corresponding to the production of specialized 

 histological elements. It did not seem to me to be conceivable 

 that the specialized idioplasm contained in the nuclei of the 

 tissue cells could re-transform itself into the initial stage of the 

 whole developmental series— that it could give up its spe- 

 cialized character and re-assume the generalized character 

 of germ-substance. I will not repeat the reasons which 

 induced me to adopt this opinion ; they still seem to me to be 

 conclusive. But let the above-mentioned theory be once 

 accepted, and there follows from it another interesting con- 

 clusion concerning the germ-cell, or at least concerning those 

 germ-cells which, like most animal eggs, possess a specific 



^ Nageli, ' Mechanisch-physiologische Theorie der Abstammungslehre,' 

 Miinchen und Leipzig, 1884. 



^ See the second and fourth Essays in the present volume. 



