THIRD PERIOD OF DEVELOPMENT. 85 



Further Lack in the section (Fig. 91, left side), 

 where the fifth mesoblastic somite is just in process 

 of formation, we see conditions similar to those which 

 we saw when the first mesoblastic somite came to 

 view. The process is only in so far a shorter one that 

 the folding at once appears much sharper. This is 

 owing to the fact that the dorsal channel is already 

 constructed during the formation of the mesoblastic 

 somites now arising, and thereby we have altered con- 

 ditions of form. 



Still further back (Fig. 01, right side) the hypoblast 

 fold is lower, and on sections which are taken near to 

 the posterior end of the embryo (Fig. 92) the more 

 sharply defined dorsal lateral edges of the hypoblast 

 form merely an intimation of the mesoblast formation. 



FORMATION OF THE XEUEAL CANAL. 



We will now turn our attention to the development 

 of the neural canal, which has been taking place 

 during the formation of the mesoblastic somites. 



While the mesoblast fold and the first mesoblas- 

 tic somite are beginning to form, the neural plate 

 is becoming separated. The latter does not stretch 

 to the whole extent of the back, but reaches only 



