98 THE AMPHIOXUS. 



neighbouring epiblast cells by the fact that the 







growth energy in the neighbouring epiblast, when the 

 yolk granules are dissolved more quickly, is a stronger 

 one than in the neural plate. This preponderating 

 growth energy of the neighbouring epiblast parts 

 leads first to a break between these and the neural 

 plate, and then to arching over the latter. 



There are thus some processes explainable by 

 ordinary mechanical conditions. We must however 

 keep in view that by far the larger number of appear- 

 ances can only be referred to such causes as we 

 without closer knowledge must comprehend under 

 the quite general name protoplasmic activity. 



SECOND SECTION OF THE THIRD PERIOD OF 



DEVELOPMENT. 

 (Formation of the Notochord.) 



In the second section of the epoch described as a 

 third period of development the body-form elongates 

 in a higher degree simultaneously with lateral com- 

 pression. 



Of internal processes are to be noticed : the further 

 increase of the mesoblastic somites and the further 

 differentiation of the same especially the processes 



