194 THE CONTINUITY OF THE GERM-PLASM AS THE 



that of the two following- segmentation nuclei. Perhaps only 

 a few groups of micellae have been displaced or somewhat differ- 

 ently arranged. But nevertheless such groups of micellae were not 

 the germs (' Anlagen ') of a second stage which pre-existed in the 

 first stage, for the two are distinguished by the possession of a 

 different molecular structure. This structure in the second stage, 

 under normal conditions of development, again brings about the 

 change by which the different molecular structure of the third 

 stage is produced, and so on. 



It may be argued that von Baer's well-known and fundamental 

 law of development is opposed to the hypothesis that the idioplasm 

 of successive ontogenetic stages must gradually assume a simpler 

 molecular structure. The organization of the species has, on the 

 whole, increased immensely in complexity during the course of 

 phylogeny : and if the phyletic stages are repeated in the ontogeny, 

 it seems to follow that the structure of the idioplasm must 

 become more complex in the course of ontogeny instead of becoming 

 simpler. But the complexity of the whole organism is not repre- 

 sented in the molecular structure of the idioplasm of any single 

 nucleus, but by that of all the nuclei present at any one time. It 

 is true that the germ-cell, or rather the idioplasm of the germ- 

 nucleus, must gain greater complexity as the organism which arises 

 from it becomes more complex ; but the individual nucleoplasms of 

 each ontogenetic stage may become simpler, while the whole mass 

 of idioplasms in the organism (which, taken together, represent the 

 stage in question) does not by any means lose in complexity. 



If we must therefore assume that the molecular structure of the 

 nucleoplasm becomes simpler in the course of ontogeny, as the 

 number of structures which it potentially contains become smaller, 

 it follows that the nucleoplasm in the cells of fully differentiated 

 tissues such as muscle, nerve, sense-organs, or glands must 

 possess relatively the most simple molecular structure ; for it cannot 

 originate any fresh modification of nucleoplasm, but can only con- 

 tinue to produce cells of the same structure, although it does not 

 always retain this power. 



AVe are thus brought back to the fundamental question as to 

 how the germ-cells arise in the organism. Is it possible that 

 the nucleoplasm of the germ-cell, with its immensely complex 

 molecular structure, potentially containing all the specific pecu- 



