424 



EMBRYOLOGY. 



Fig. 237. Median section through the head of a 

 Rabbit embryo 6 mm. long, after MIHALKOVICS. 



rh, Pharyngeal membrane ; hp, place whence the 

 hypophysis develops ; h, heart ; kd, cavity of the 

 head-gut ; ch, chorda ; v, ventricle of the cere- 

 brum ; v", third ventricle, that of the between- 

 brain ; *, fourth ventricle, that of the hind- and 

 after-brain ; cTc, central canal of the spinal cord.'* 



the part of the base of the brain lying behind it, but afterwards an 



acute angle (figs. 235, 238). 

 In consequence of this, the 

 vesicle of the mid - brain 

 (fig. 235 mh) conies to lie 

 highest, and forms a promi- 

 nence, which causes a great 

 protrusion of the surface of 

 the embryo and is known 

 as the parietal prominence 

 (fig. 158 ). 



The nuchal flexure, which 

 makes its appearance at the 

 boundary between medulla 

 oblongata and spinal cord, 

 is less prominent (fig. 235 

 nb). It produces in the 

 embryos of the higher Ver- 

 tebrates a curvature which also projects outward, the so-called 



nuchal prominence 



(fig. 158). 



The third curva- 

 ture, which has been 



designated by KOL- 



LIKER as the pontal 



flexure (fig. 239 bb), 



because it arises in 



the neighborhood 



of the future pons 



Varolii, is, on the 



contrary, very 



marked. It is 



further distinguished 



from the two other 



curvatures described, 



by the fact that its 



convexity is not di- 

 rected toward the 



back of the embryo, 



but toward its ventral side. It is formed between the floor of the 

 * [For terminology of the regions of the brain, see footnote, p. 282.] 



Fig. 238. Median sagittal section through the head of a Chick 

 incubated four and a-half days, after MIHALKOVICS. 



SH, Parietal prominence ; sv, lateral ventricle ; V s , third 

 ventricle ; v\ fourth ventricle ; Sw, aqueduct of SVLVIUS; 

 gh, vesicle of the cerebrum ; zh, between -brain ; mh, mid- 

 brain ; kli, cerebellum ; zf, pineal process (epiphysis) ; 

 Tip, pocket of the hypophysis (pouch of RATHKE) ; cli, 

 chorda ; ba, basilar artery. 



