128 The Science of Life. 



divisions ", whereby the germ-cells were prepared for 

 their subsequent union in fertilization and the number 

 of chromosomes was kept constant in the species. If 

 it were not for the preparatory reduction the number of 

 nuclear elements or chromosomes would be doubled at 

 each fertilization. By the brilliant work of Platner, 

 Boveri, O. Hertwig, and many others, for animals, of 

 Guignard, Strasburger, and others, for plants, this fact 

 at least seems to be securely established amidst a maze 

 of uncertainties, that in the history of both male and 

 female germ-cells the number of chromosomes is reduced 

 to one-half of the number characteristic of the body-cells 

 of the species. 



Another embryological corollary of the cell-doctrine 

 is that development implies cell-formation, or that the 

 The Mode of fi rst step after fertilization is the cleavage 

 Development. or segmentation of the ovum. 



In 1826 PreVost and Dumas had given the first defi- 

 nite description of the cleavage of the frog's egg, showing 

 that it first divides into two cells, then into four, then 

 into eight, and so on; but the full import of the fact 

 was not realized until later. Thus Schwann and 

 Schleiden believed that cells might arise either by the 

 division of a pre-existing mother-cell or by a process of 

 "free cell-formation". In the latter case, as Wilson 

 says, new cells were supposed to crystallize out, as it 

 were, within a formative or nutritive substance, termed 

 the " cytoblastema ". "It required many years of re- 

 search to show that ' free cell-formation ' was a myth, 

 though this had been suggested by many of Schwann's 

 immediate followers, and though Virchow had, in 1855, 

 positively maintained the universality of cell-division, 

 contending that every cell is the offspring of a pre- 

 existing parent cell. He summed up his position in the 

 aphorism, omnis cellula e cellula." 



But Virchow's conclusion required detailed corro- 

 boration, and this was afforded by the early studies on 

 ovum-segmentation and tissue-formation (histogenesis) 

 associated with the names of Kolliker, Reichert, Remak, 

 and many others. 



Moreover, in combination with the facts beginning to 



