E PI GENESIS OR EVOLUTION. 115 



division of the original single cell a number of cells are 

 produced which do not remain like one another, but at fixed 

 times in every species take up a fixed position and give rise 

 to further cells which go through a perfectly regular set of 

 form changes, resulting in the formation of tissues and 

 organs in definite and determined relations to one another. 

 These relations taken together form the character of the 

 species. The process is one not only of cell multiplication, 

 but also of cell differentiation, and the differentiation during 

 the course of ontogeny must be effected in one of two 

 different ways. Either the original cell, the ovum, divides 

 quantitatively, so that each resulting cell is composed of 

 similar material and only goes through its subsequent form 

 changes as the result of its position and the reaction of the 

 cells contiguous to it, or the ovum divides qualitatively and 

 the resulting cells are composed of unlike material, the sub- 

 sequent form changes being the result of the unlikeness of 

 the material of which they are composed. The latter of 

 these two alternatives is preferred, partly because in a 

 number of instances the differentiation of the cells is 

 apparent from the first cleavage onwards. Of the two 

 first blastomeres, each has a distinct prospective career, 

 each differs in its essential characters from the other. With 

 the second, third, and fourth cleavages, further differentia- 

 tion is effected, and the individual cells are stamped, as it 

 were, with their particular characters from the very com- 

 mencement. These are qualitative changes, and are not 

 to be accounted for by the action of any external agencies ; 

 the determining force must reside within the cell, and must 

 be the result of a different constitution effected by the separa- 

 tion of unlike materials which were present in the ovum. And 

 this view is largely borne out (according to the argument) by 

 experiments which have been made on segmenting ova ; it 

 has been shown, for instance, that if the two primary blasto- 

 meres of a frog's ovum be separated by mechanical means, 

 each will continue to develop, not as a whole, but as a half 

 organism, one blastomere giving rise by further subdivision 

 to the right half, the other to the left half of the embryo. 

 Clearly, then, the first division does not affect the mass 



