FOURTH PERIOD OF DEVELOPMENT. 147 



61). This posterior thickened end of the medullary 

 tube forms the undifferentiated material which, on 

 the completion of the growth and the further mul- 

 tiplication of the myotomes, is employed on the 

 formation of new sections of the neural canal. 



This terminal section of the neural canal, which 

 belongs to the undifferentiated region curves during 

 this period of development ventralwards round the 

 posterior end of the notochord, as has been well shown 

 by Kowalevsky, the communication with the meseii- 

 teron still remaining. 



The thickening of the anterior end of the neural 

 canal is more prominent too in this period of devel- 

 opment. The latter is also towards the anterior 

 sharply separated from the thinner epithelium of the 

 surface of the body. The brain-like thickening of the 

 neural canal is readily to be observed on a side view 

 of the embryo. It is seen how owing to it the 

 notochord experiences a very considerable indentation 

 (Figs. 60, 61). It may be recognised also how the 

 central canal, which is here somewhat wider, opens 

 outwards towards the front into a funnel-shaped de- 

 pression of the outer surface. The brain-s welling 

 can be shown still better in transverse sections. 



