THIRD PERIOD OF DEVELOPMENT. in 



Later on we see in the sections which correspond 

 to the same region, the folding already introduced, 

 while the formation of the notochord in the region 

 of the mesoblastic somite of the same embryo has 

 advanced considerably further. Figs. 95 and 96 answer 

 more or less to the same place of the embryo as Fig. 

 86, taken from an earlier stage. In the sections 

 belonging to the later stage, Figs. 95 and 96, the 

 representation is remarkably different, owing to the 

 fact that the closing of the neural canal has now 

 advanced further, and also to the fact that the 

 anterior prolongations of the first mesoblastic somite 

 are cut by the section, while on Fig. 86 nothing was 

 to be seen of the formation of the rnesoblast. Quite 

 in the front, the condition of the hypoblast in this 

 stage is similar to that in Fig. 104 (which belongs to 

 a later stage). There is still no trace of the channel 

 of the notochord to be found. 



We should now observe the series of sections from 

 the anterior end of a still older embryo. There we see 

 that in the region which corresponds to Figs. 95-93, 

 the slit of the notochord is now entirely vanished 

 (Fig. 107). Further forward, however, we still con- 

 tinue to find the open channel (Fig. 106), which 



