THE GERM-PLASM 79 



by variations in the determinants of similar origin taking place 

 in different directions. The mere addition of a new ontogenetic 

 stage can very easily be conceived without an increase occurring: 

 in the determinants of the id ; but as soon as the double number 

 of cells which are present in the new idic stage have to become 

 differentiated in various ways, the differentiation must be pre- 

 ceded by a doubling of the determinants in the id of germ- 

 plasm. A higher degree of differentiation will therefore be 

 primarily connected with an increase in the number of cells of 

 which the organism is constituted. It is known that the extreme 

 prolongation of development, due to the constant addition of 

 new generations of cells at the end of ontogeny, can be neutral- 

 ised by the abbreviation and reduction of the ontogenetic stages : 

 this process may be also to some extent understood if we trace 

 it to its origin in the structure of the idioplasm. The reduction 

 in the number of generations of cells from two or more to one, 

 depends on the fact that the process of multiplication and rear- 

 rangement of the determinants takes place more rapidly during 

 these particular stages, than does that of cell-division ; so that 

 several idic stages, each of which formerly characterised a 

 particular stage of the cell, pass into one another during the 

 same stage of the cell. The respective idic stages have not 

 here disappeared completely' : they only follow one another 

 more rapidly, and therefore disappear as recognisable stages in 

 development. 



In lowly organised beings the differentiation of the body may 

 become increased by a simple reduction of the iiereditary parts 

 or deterf/iifiates, without an increase taking place in the cell- 

 generations. If a determinant which controls a region con- 

 sisting of a hundred cells divides into two, each of which only 

 controls fifty cells, the two resulting groups of cells can vary 

 independently of each other from this point onwards, and may 

 give rise to very different structures. In this way a continued 

 division of the determinants, and consequently also a constantly 

 increasing differentiation of the species, may occur, without 

 necessitating an increase in the total number of cells present in 

 ontogeny. 



Each additional differentiation denotes an increase in the 

 degree of organisation. But the phyletic development of the 

 organism is by no means invariably connected with an increase, 

 or. in fact, with any other change in the degree of organisation. 



