THE LAW OF BIOGENESIS 83 



of chromosomes as a result of this method of di- 

 vision. 



In these external and internal changes in the 

 embryo there is a type of energy at work found 

 only in living protoplasm which is explained in part 

 in the chapter "What Makes Our Body Go." 

 Some additional facts are explained in the chapter 

 on Heredity and much remains that is not under- 

 stood. 



The next stage in the division of the salamander 

 embryo produces four cells. (Fig. 26.) The 

 later stages are irregular in the order of their ap- 

 pearance and unequal in the size of the cells pro- 

 duced. (Fig. 26 C-F.) Such early irregularity 

 in the shape of cells in the embryo is not a constant 

 feature in the development of animals but is pe- 

 culiar to the salamanders and some fishes. 



As the growth of these cells continues, many 

 cells are produced, the size of the embryo in- 

 creases and takes on a specific shape. Finally a 

 conspicuous groove appears on one surface (Fig. 

 26 G-I), which gradually changes until it becomes 

 enclosed within the body of the embryo. One can 

 now recognize a head and tail region at this stage. 

 What is this groove on the outside of the embryo? 

 It is the first stage in the growth of the nervous 

 system of this salamander. The nervous system, 

 then, is produced by the same kind of cells that 



