108 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



The mid-brain is bounded in front and behind by grooves, and its 

 transverse diameter is rather less than that of either fore-brain or 

 hind-brain. The hind-brain forms by far the largest part of the 

 whole brain, and passes without distinct demarcation into the spinal 



Op. 



Fig. 13. Section through optic vesicle (Op.). 

 Fb., wall of fore-brain ; EC., external ectoderm. 



part of the medullary tube. The hind-brain is of interest in that it 

 shows certain neuromeres very clearly ; these neuromeres form folds 

 involving the whole thickness of the lateral wall of the hind-brain, 

 and are convex externally, with corresponding concavities internally 



Hb. 



Mfc 



Fig. 14. Section through head of 

 embryo, x 50. 



Fb., fore-brain; Mb., mid-brain; //., 

 ///. and IV., 2nd, 3rd and 4th 

 neuromeres of hind-brain ; Op., 

 optic vesicle; Au., auditory pit; 

 G.a.f., ganglion acustico-facialis. 



Fig. 15. Section through region of first 

 visceral cleft, x 50. 



Ft,, fore-brain; Hb., hind-brain; Op. , optic 

 vesicle; Ph., pharynx; I.V.Cl., first 

 visceral cleft ; Au., auditory pit ; 

 d.Ao., dorsal aorta; Cli., notochord ; 

 Ao.l., first aortic arch vessel. 



(Fig. 14). The neuromeres are seven in number, as has already been 

 noted by Broman (3) in a human embryo of 3 mm., and more recently 

 by Thompson in the twenty-three somite embryo and Ingalls in an 

 embryo of 4'9 mm. (6). The first neuromere is the smallest, and does 



