424 



EMBRYOLOGY. 



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) comes 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 s). 



The nuchal flexure, which 

 makes its appearance at the 

 boundary between medulla 

 oblongata and spinal cord, 

 is less prominent (fig. 235 

 Ti-6). It produces in the 

 embryos of the higher Ver- 



ch 



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 3 , third ventricle, that of the between- 

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

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



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 rioor 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 3 , third 

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

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

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

 hp, pocket of the hypophysis (pouch of RATHKE) ; c/i, 

 chorda ; ba, basilar artery. 



