xiv ELEMENTS OF VERTEB RAT I. KM BRYOLOGY 459 



cord. The main regions of the brain are beginning to be marked off and 

 in many embryos two distinct constrictions mark off thalamencephalon, 

 mesencephalon and rhombencephalon. The brai? . !,!, has a 



pu 



ms 



mf. 





av: 



FIG. 192. 



Blastoderm with Fowl embryo with about 10 or u mesodenn segments, .a.v, Vascular area ; 

 f.g, fore-gut ; h, head ; m.f, medullary fold ; m.s, mesodenn segment ; o.r, optic nidinn-nt ; 

 pa, proamniou a part of the blastoderm into which the mesoderm has not yet spread. 



characteristic hammer shape, the wall of the thalamencephalon projecting 

 laterally on each side to form the optic rudiment (Fig. 192, o.r). 



Important changes have taken place in the mesoderm. The originally 

 single sheet of mesoderm has, except towards its outer boundary and in 



