IV.] FOUNDATION OF A THEORY OF HEREDITY. 21 7 



follows from this conclusion that my theory is further, and as 

 I believe strongly, supported, by the phenomenon of the ex- 

 pulsion of polar bodies, which has remained inexplicable for so 

 long a time. 



For if the specific molecular structure of a cell-body is 

 caused and determined by the structure of the nucleoplasm, 

 every kind of cell which is histologically differentiated must 

 have a specific nucleoplasm. But the egg-cell of most animals, 

 at an}'' rate during the period of growth, is by no means an 

 indifferent cell of the most primitive type. At such a period 

 its cell-body has to perform quite peculiar and specific func- 

 tions ; it has to secrete nutritive substances of a certain chemi- 

 cal nature and physical constitution, and to store up this food- 

 material in such a manner that it may be at the disposal of the 

 embryo during its development. In most cases the egg-cell 

 also forms membranes which are often characteristic of par- 

 ticular species of animals. The growing egg-cell is therefore 

 histologically differentiated : and in this respect resembles a 

 somatic cell. It may perhaps be compared to a gland-cell, 

 which does not expel its secretion, but deposits it within its 

 own substance '. To perform such specific functions it requires 

 a specific cell-body, and the latter depends upon a specific 

 nucleus. It therefore follows that the growing egg-cell must 

 possess nucleoplasm of specific molecular structure, which 

 directs the above-mentioned secretory functions of the cell. 

 The nucleoplasm of histologically differentiated cells may be 

 called histogenetic nucleoplasm, and the growing egg-cell must 

 contain such a substance, and even a certain specific modifica- 

 tion of it. This nucleoplasm cannot possibly be the same as 

 that which, at a later period, causes embryonic development. 

 Such development can only be produced by true germ-plasm 

 of immensely complex constitution, sueh as I have previously 

 attempted to describe. It therefore follows that the nucleus of 

 the egg-cell contains two kinds of nucleoplasm : — germ-plasm 

 and a peculiar modification of histogenetic nucleoplasm, which 

 may be called ovogeiietic nucleoplasm. This substance must 



\} Such gland-cells are known in both animals and plants. See W. 

 Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting 

 cells of Blechnum occideiitale L., and Osmunda regalis L., ' Annals of 

 Botany,' vol. i. p. 49.— S. S.] 



