THE CELL 95 



and providing protection for the embryo and is provided 

 with only sufficient cytoplasm to form a locomotor ap- 

 paratus by which it seeks the ovum. It is therefore very 

 small and performs active movements. The plant ovum 

 which is usually known as the oosphere shows the same 

 general features as that of animals. The flagellum or tail 

 is merely a locomotor organ, which plays no part in fer- 

 tilization. Its most characteristic feature is the axial fila- 

 ment, which is composed of a large number of parallel 

 fibrillae like a muscle fibre. Both the ova and spermato- 

 zoon take their origin from cells known as primordial 

 germ cells, which become clearly distinguishable from the 

 somatic cells at early period of development and are at 

 first exactly alike in the two cases. Moreover, from the 

 outset the progenitors of the germ cells differs from the 

 somatic cells not only in the greater size and richness of 

 chromatin of its nuclei but also in its mode of mitosis, 

 for in all those blastemers destined to produce somatic 

 cells, a portion of the chromatin is cast out into the cyto- 

 plasm where it degenerates and only in germ cells is the 

 sum total of the chromatin retained. Only the germ cells 

 receive the sum total of the egg chromatin handed down 

 from the parent. All of the somatic cells contain only a 

 portion of the original germ substance. The original 

 nuclear constitution of the fertilized egg is transmitted as 

 if by law of primogeniture, only to the one daughter cell 

 and by this again to one and so on. While in the other 

 cells, the chromatin in part degenerates, in part is trans- 

 formed so that all of the descendants of these side branches 

 receive small reduced nuclei. The number of chromo- 

 somes arising from the germ nuclei is always the same in 

 both and is one-half the number characteristic of the tis- 

 sue cells of the species. The two nuclei do not fuse, but 



