374 THE BABBIT. 



bend at the junction of the spinal cord and brain, between the 

 reference lines HP and OI in Fig. 151, is very characteristic of 

 mammalian embryos at this stage ; while the sharp bend at the 

 base of the mid-brain is quite as conspicuous as in the earlier 

 stage. 



The general relations of the brain to the head are much the 

 same in the rabbit embryo as in the chick. In the early stages 

 (Figs. 147, 148, and 150) the brain forms practically the whole of 

 the head, and determines its shape almost exclusively ; but in 

 the later stages (Figs. 149, 151), as the parts of the face assume 

 definite shape, and grow forwards to form the snout, the brain 

 becomes thrown more and more on to the dorsal surface, and 

 ultimately to the posterior part of the head, and takes a much 

 less prominent share in determining the general contour. 



In dealing with the several parts of the brain it will be con- 

 venient to commence with the thalamencephalon, and the struc- 

 tures developed from, or in connection with it. 



The thalamencephalon (Fig. 150, BF) is the anterior cerebral 

 vesicle, or fore-brain, of the early stages (cf. Figs. 145, 146). 

 From it the optic vesicles arise as lateral outgrowths ; these 

 appear very early, and attain some size before the roof of the 

 fore-brain is closed (Fig. 145) ; their further development, and 

 their conversion into the essential parts of the eyes, will be 

 described in the next section of this chapter, p. 387. 



The side walls of the thalamencephalon thicken very quickly, 

 to form the optic thalami (Fig. 155, uu) ; and, owing to this thick- 

 ening of its sides, the central cavity, or third ventricle, becomes 

 reduced to a vertical cleft, very narrow from side to side. The 

 anterior wall of the thalamencephalon remains thin, as the lamina 

 terminalis (Fig. 151, BT), lying between the roots of the two 

 cerebral hemispheres. 



The roof of the thalamencephalon remains thin, consisting in 

 the greater part of its extent of a single layer of epithelial cells, 

 devoid of nervous elements of any kind. From this roof, rather 

 behind the middle of its length, the pineal body arises about the 

 twelfth day, as a hollow median papilla (Fig. 150, risr). This 

 rapidly lengthens, forming a tubular and backwardly directed 

 diverticulum of the brain. Its distal end dilates (Fig. 151, PX), 

 to form a slightly expanded vesicle, from the sides of which 



