ANIMAL EVOLUTION 19 



the body and what organs and tissues the descendants 

 of that cell give rise to. 



It has thus become possible to construct a large number 

 of cell-lineages, each of which can be expressed in the 

 form of a genealogical tree. 



The tracing out of cell-lineages is a matter of pure 

 observation, and the whole result of the work has been 

 to show, what might have been predicted from our 

 knowledge of the building up of embryo from the egg 

 and the adult from the embryo by a repeated process 

 of cell-division, that in a normal course of development, 

 particular cells having particular positions in the embryo 

 invariably give rise to the same organs in the larva or 

 in the adult. This observed fact could be interpreted 

 just as well on Herbert Spencer's principles as on Weis- 

 mann's, for it could be urged that the fate of any particular 

 cell occupying a particular position in the embryo 

 depended, not on any qualities inherent in itself, that 

 is to say on the particular kind of material of which it 

 is composed, but upon the forces to which it is subject, 

 because of its relation to other cells and to the external 

 environment. 



But reasoning on the results of simple observation 

 can only lead to hypothetical explanations. Experiment 

 alone can decide the questions at issue. The earliest 

 experiments seemed to support Herbert Spencer's views. 

 Driesch, and after him other investigators, found that, 

 in a number of animals, the first two, or four, or even 

 eight or sixteen, cells formed by the earlier divisions of 

 the germ-cell, might be separated from one another, 

 and that each would, after separation, segment as if 

 it was an entire ovum and give rise to a normal larva, 

 having all the specific characters of a normally developed 

 larva, but of reduced size. O. Hertwig, Driesch, and 

 others showed that the mutual positions of the blasto- 

 meres might be altered by pressure and other methods, 

 and yet the altered embryo when released and kept 



