THIRD PERIOD OF DEVELOPMENT. 127 



ventral view (Fig. 53) of the embryos, as in the trans- 

 verse sections (Figs. 105, 106, and Figs. 113, 114). 



In the transverse sections of the older embryos at 

 this period of development, we are struck by the 

 relation of these folds to the prolongation of the 

 anterior end of the mesqblast of the first mesoblastic 

 somite. We see that this prolongation inserts itself 

 dorsally between the hypoblast fold and the nervous 

 system. 



These dorsal folds separate later from the alimentary 

 canal, and show in the further development a very 

 remarkable unsymrnetrical condition. We will gain 

 some acquaintance with these processes in the next 

 period of development. I will only mention here, that 

 the unsymmetrical formation begins as early as in the 

 stages with nine, ten, and eleven mesoblastic somites. 



THE EPITHELIUM. 



We will further add some remarks with regard to 

 the changes in the outer epithelial layer. In view 

 of the extension which the embryo experiences during 

 this last period of development, the epithelium 

 becomes continually lower, so that the individual 

 cells change continually more and more from the form 



