THIRD PERIOD OF DEVELOPMENT. 79 



is accompanied by a sharp furrow running trans- 

 versely over the mesoblast fold on its outer as well as 

 on its inner surface. It is this transverse and posterior 

 boundary of the first mesoblastic somite which speci- 

 ally attracted our attention in the living object. A 

 third indentation forms the side boundary of the 

 mesoblastic somite. This is only to be observed on 

 transverse sections (Fig. 74). 



The next mesoblastic somites are formed by means 

 of a similar transverse fold, and the posterior boundary 

 of the mesoblastic somite, as well as the indenta- 

 tion which forms the lateral boundary, are repeated 

 in quite a similar way (Figs. 42-45). The process 

 however of the formation is, as we shall subsequently 

 see, a very much shorter one when the latsr 

 mesoblastic somites begin to appear. 



During the formation of the second and third 

 mesoblastic somites the neural plate continually 

 sinks in deeper between the two mesoblast folds. 

 Thereby the longitudinal folding as well in the 

 region of the mesoblastic somites as in that of 

 the posterior uiidifFerentiated mesoblast foundation, 

 is much more sharply marked than can be seen 

 from the series of transverse sections of Table VII. 



