346 CLASSICS OF MODERN SCIENCE 

 the Hydroids, on which my theory of the continuity of the germ-plasm 

 has been chiefly based, alternation of generations is the most impor- 

 tant means of propagation. 



THE SIGNIFICANCE OF THE POLAR BODIES 



We have already seen that the specific nature of a cell depends upon 

 the molecular structure of its nucleus ; and it follows from this con- 

 clusion that my theory is further, and as I believe strongly, sup- 

 ported, by the phenomenon of the expulsion 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 dififerentiated must have a specific nucleoplasm. 

 But the egg-cell of most animals, at any 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 

 functions ; it has to secrete nutritive substances of a certain chemical 

 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 particular 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 modification 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 the true germ-plasm of 

 immensely complex constitution, such as I have previously attempted 

 to describe. It therefore follows that the nucleus of the egg-cell 

 contains two kinds of nucleoplasm: — germ and a peculiar modi- 



