26 HERBERT SPENCER AND 



troch cells p. It divides into /)^ and P'^, and each of these 

 divides again into />", f^"^ and p^^, p^^. If in any embryo 

 p is isolated on its first appearance, it divides twice, 

 and gives rise to four cells each bearing a tuft of cilia. 

 If ^1 and p"^ are isolated, each divides once, giving rise to 

 two cells with a tuft of cilia. If any of the cells p^^, p^^, 

 pzi^ pzi^ is isolated it develops a tuft of cilia, but will not 

 divide. The number of divisions is predetermined, and 

 that number is fulfilled whether the cell remains a part of 

 the aggregate or whether it is removed from it. 



The incident forces and the mutual actions and reactions 

 must be very different in the two cases, but the specific 

 number of divisions are gone through and the specific 

 form is attained all the same. 



Clearly, in such cases the destiny of a cell is predeter- 

 mined, and is not a function of its position in the cell 

 aggregate. One cannot say with Driesch * jeder Theil 

 kann jedes '. 



It is, of course,* impossible for me to give you a 

 hundredth part of the available evidence ; but this 

 much, I think, emerges clearly from what I have told 

 you. 



AU ova or eggs contain specific substances necessary for 

 the formation of particular regions of the future animal. 

 In the course of segmentation these substances are 

 segregated from one another in a progressive manner, 

 so that in the end particular kinds of substance are isolated 

 in particular cells, derived from division of the original 

 germ-cell. Cell-division therefore, in ontogeny, is the 

 expression of the sorting out and separation of the 

 materials contained in the germ. 



This process of segregation sets in earlier in some 

 species than in others. In some cases, as in Dentalium, 

 it begins with the very first division of the germ-cell. 

 In other cases the segregation is deferred until a numiber 

 of cells are present (as is shown by the fact that sometimes 

 any one of the first sixteen blastomeres is capable of 



