THEIE SIGNIFICANCE IN HEREDITY. 341 



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 de- 

 termines the course of embryonic 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 2 . Germ-plasm is only 

 a certain 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, corre- 

 sponding to the production of specialized histological elements. 

 It did not seem to me to be conceivable that the specialized idio- 

 plasm 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 specialized 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 conclusion 

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

 which, like most animal eggs, possess a specific histological cha- 

 racter. For obviously, such a character presupposes the existence 

 of an idioplasm with a considerable degree of histological special- 

 ization, which must be contained in the nucleus of the egg-cell. 

 We know, on the other hand, that when its growth is complete, 

 after the formation of yolk and membranes, the egg contains 



1 Nageli, ' Mechanisch-physiologische Theorie der Abstammungslehre,' Munchen 

 und Leipzig, 1884. 



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



