DEVELOPMENT OF THE NERVOUS SYSTEM. 



43 



fundamental importance, except as the adult form of certain parts 

 of the brain and the position of some of its nuclei are doubtless 

 determined by the influence of the flexures in these early stages. 

 As the head of the embryo straightens out and as the growth of 

 the other organs relaxes the pressure upon the brain, the flexures 

 tend to disappear and they are to a large extent obliterated in the 

 brains of lower vertebrates (compare Figs. 2, 11, 12, 158). 



Fig. 22. — The development of the optic vesicles in Amblystoma punctatum. 

 Fig. 22 A, should be compared with a similar figure by Eycleshvmer. In B the 

 germinal cells in process of division are shown by small black figures. 



In the meantime the rapid growth of certain parts of the brain 

 while the flexures are still present, results in deepening the con- 

 strictions between certain of the neuromeres and the obliteration of 

 others, and produces the prominent secondary segments which have 

 been described in the previous chapter. Only a brief summary 

 of these changes can be given here, since a fuU description would 



