510 THE HUMAN EMBRYO. 



eiice line AN, marks the junction of the brain and spinal cord ; 

 the highest point of the cephalic convexity, close to the refer- 

 ence line HD, is the region of the mid-brain ; and the part in 

 front of this is the fore-brain, which is already flexed ventral- 

 wards. 



The third week. By the fifteenth day (Figs. 197, p. 489, 

 and 232, p. 545) the neural canal is closed along its whole 

 length, except at the extreme hinder end ; the several divisions 

 of the brain fore-brain, mid-brain, and hind-brain are well 

 established; and cranial flexure is strongly marked, a sharp 

 bend of about 90 degrees taking place opposite the mid-brain, 

 by which the fore-brain is brought down to the under surface of 

 the head. 



The fore-brain is of considerable length ; its most anterior 

 part is the vesicle of the hemispheres, a short, rounded, and com- 

 paratively inconspicuous dilatation, which as yet shows no trace 

 of division into right and left hemispheres. The thalamen- 

 cephalon, or fore-brain proper (Fig. 232, BF), is long, and com- 

 pressed laterally ; from its sides arise the optic vesicles, BO, 

 which project outwards and slightly backwards, and are already 

 constricted at their bases to form the optic stalks. The floor of 

 the thalamencephalon is produced downwards behind the optic 

 stalks into a shallow pit, the infimdibulum. 



The mid-brain, BM, is small and rounded ; it is separated by 

 a constriction from the fore-brain in front, and by a much 

 sharper one, the isthmus, from the hind-brain. 



The hind-brain is the widest as well as the longest part of 

 the brain ; it is widest in front, and gradually tapers posteriorly 

 as it passes into the spinal cord. The roof of the hind-brain 

 is very thin, except at its anterior end, where a slightly 

 thickened transverse band, BL, marks the commencement of the 

 cerebellum. 



During the third week the brain rapidly increases in size, 

 and by the end of the week has attained the proportions shown 

 in Fig. 215. The several divisions of the brain are more dis- 

 tinctly marked off from one another, and the vesicle of the hemi- 

 spheres, BS, and the cerebellum, BL, are more conspicuous than 

 before. 



The cervical flexure, by which the entire head is bent ventral- 



