THIRD PERIOD OF DEVELOPMENT. 109 



namely, in the fifth mesoblastic somite (Fig. 101) ; 

 and still further back, in the sixth, the cavity of 

 the fold may be seen (Fig. 102). In the neighbour- 

 hood of the posterior mesoblast folds the middle 

 hypoblast plate is still completely flattened (Fig. 103) ; 



thus the formation of the notochord has not yet 

 begun. 



The advance of the differentiation from front to 

 back, as characteristic for the metameric type, finds 

 its expression here. 



In the formation of the later mesoblastic somites 

 the process of the formation of the notochord appears 

 abbreviated, as we shall later on explain still more 

 fully. 



Of special interest is. too, the formation of the 

 notochord in the anterior part of the body, namely, 

 that situated in front of the mesoblastic somite 

 region, especially having regard to the fact that the 

 Amphioxus is distinguished from all vertebrates and 

 also from the Ascidians, thus from the whole race of 

 the Chordata, by means of the notochord, which 

 reaches right into the anterior end. 



We will here mention that as a matter of fact in 

 the region of the first mesoblastic somite the develop- 



