FOUNDATION OP A THEORY OF HEREDITY. 205 



The fact, already often mentioned, that in most higher, organisms 

 the separation of germ-cells takes place later, and often very late, 

 at the end of the whole ontogeny, proves that the time at which 

 this separation of the two kinds of cells took place, must have been 

 gradually changed. In this respect the well-established instances 

 of early separation are of great value, because they serve to connect 

 the extreme cases. It is quite impossible to maintain that the 

 germ-cells of Hydroids or of the higher plants, exist from the 

 time of embryonic development, as indifferent cells, which cannot 

 be distinguished from others, and which are only differentiated at a 

 later period. Such a view is contradicted by the simplest mathe- 

 matical consideration ; for it is obvious that none of the relatively 

 few cells of the embryo can be excluded from the enormous increase 

 by division, which must take place in order to produce the large 

 number of daughter individuals which form a colony of polypes. 

 It is therefore clear that all the cells of the embryo must for a long 

 time act as somatic cells, and none of them can be reserved as 

 germ-cells and nothing else : this conclusion is moreover confirmed 

 by direct observation. The sexual bud of a Coryne arises at a 

 part of the Polype which does not in any way differ from sur- 

 rounding areas, the body wall being uniformly made up of two 

 single layers of cells, the one forming the ectoderm and the other the 

 endoderm. Rapid growth then takes place at a single spot, and 

 some of the young cells thus produced are transformed into germ- 

 cells, which did not previously exist as separate cells. 



Strictly speaking I have therefore fallen into an inaccuracy in 

 maintaining (in former works) that the germ-cells are themselves 

 immortal ; they only contain the undying part of the organism 

 the germ-plasm ; and although this substance is, as far as we 

 know, invariably surrounded by a cell-body, it does not always 

 control the latter, and thus confer upon it the character of a 

 germ-cell. But this admission does not materially change our 

 view of the whole subject. We may still contrast the germ-cells, 

 as the undying part of the Metazoan body, with the perishable 

 somatic cells. If the nature and the character of a cell is deter- 

 mined by the substance of the nucleus and not by the cell-body, 

 then the immortality of the germ-cells is preserved, although only 

 the nuclear substance passes uninterruptedly from one generation 

 to another. 



