74 THE AMPHIOXUS. 



the neural plate, the dorsal surface at the same 

 time again assuming a rounded form. 



With regard to the details in the formation of the 

 neural canal, which are not to be recognised in the 

 living object, subsequent information will be given 

 by means of the other methods. 



It is at about this stage (Fig. 39) that the embryos 

 leave the egg membrane. This is however often 

 burst somewhat earlier, while in other individual 

 cases embryos are again to be found whose third 

 mesoblastic somite is already in course of formation 

 inside the egg membrane. The bursting of the latter 

 seems to me in part at least to be brought about 

 by the ever increasing rotation of the embryo. 

 Perhaps too we have to deal with a gradually altered 

 consistency of the egg membrane. When further 

 distended, it is generally seen to be split, and to have 

 fallen asunder into two parts. 



The movement of the embryos inside the egg mem- 

 brane, and also after leaving it, is a quite peculiar one. 

 They swim continually with the anterior end of the 

 body bent forwards, and revolve at the same time on 

 their axis, this revolution always following the same 

 direction from right to left. Thus all the points of the 



