THIRD PERIOD OF DEVELOPMENT. 73 



the posterior and undifferentiated part of the meso- 

 blast fold, while towards the anterior, and in the 

 direction of the ventral side, it passes into the 

 wall of the archenteroii without any distinctly 

 visible boundary in the living object. The lumen 

 too of the first mesoblastic somite is still connected 

 with the lumen of the archenteron by means of a 

 wide opening. 



The second mesoblastic somite is separated by 

 means of a second boundary which arises in a similar 

 way (Figs. 37, 38, 39). 



During the formation of the two first mesoblastic 

 somites the neural plate, forming the floor of 

 the neural canal, is arched over by the edges of the 

 latter. 



This is however a process which can only be 

 followed with difficulty in the living object. It can 

 nevertheless be seen in these stages that the gas- 

 trula-mouth is not open any more, and optical tran- 

 verse sections teach us that the neural plate has 

 separated underneath the superficial layer of cells. 

 The dorsal surface of the embryo is furthermore still 

 continually deepened in the form, of a channel, and 

 it is only gradually that its epithelium is raised above 



