408 



Fig. 4 and sketch C are camera drawings of a living embryo of 

 the same age as the one just described. A thin transparent un- 

 segraented roof covering the encephalic segments, has been formed. 

 The segments of the medulla are very distinct, the segmental grooves 

 passing directly across the floor of the 4th ventricle to be continued, 

 in the same transverse plane, on the external surface both laterally 

 and ventrally. 



As was observed in younger embryos, the dorsal groove (/") which 

 marks the anterior limit of the medulla, continues to be deeper than 

 any of the other dorsal constrictions. The first two grooves cephalad 

 to this, mark respectively the anterior limit of the cerebellum and the 

 second segment of the mid-brain, and can be traced around the en- 

 cephalon in a manner similar to the grooves of the medulla. The 

 three grooves anterior to these are less distinct. The posterior limits 

 of segments 1, 2 and 3, could be detected only along the dorsal en- 

 cephalic crest. In view of the fact that these grooves are very distinct 

 on both the inner and the outer surfaces of the encephalon, in divided 

 and dissected embryos of this age as will be shown later, I have 

 concluded my failure to trace them ventrally along the sides of the 

 brain in living embryos, was due to the presence of the eyes that are 

 closely applied to the brain wall of this region, and also to the 

 cephalic mesoderm that forms a dense tissue just dorsal to the optic 

 vesicles. 



Figs. 5 and 6 present dorsal and lateral views respectively of 

 the cephalic portion of a living embryo 23 days old, with 25 somites. 



