THE ORGANS OF THE OUTER GERM-LATER. 



423 



and evaginations subsequently become still more sharply marked 

 off from one another, owing to the alteration of their positions. 



Fig. 235. — Brain of a human embryo of the third week i,Lg). Fioflle reconstruction. After His, 

 gh. Cerebral vesicle ; zh, betweeu-brain vesicle ; mh, mid-brain vesicle ; kh, nfi, vesicles of the 



cerebellum and medulla oblongata ; au^ optic vesicle ; gb, auditory vesicle ; tr, inf undibulum ; 



r/f area rhomboidalis ; nb, nuchal flexure ; kb, cephalic flexure. 



At the beginning the three brain-vesicles formed by the first 



constrictions lie in a straight line one behind the other (fig. 87) and 



above the chorda dorsalis ; the latter extends 



only as far as to the anterior end of the mid- 

 brain vesicle, where it tapers to a point. But 



from the moment when the optic vesicles begin 



to be cottstricted off, the three primary vesicles 



shift their positions in such a way that the 



longitudinal axis uniting them undergoes sharp, 



characteristic folds, which are distinguished as 



the cephaKe, pontal, and nuchal flexures (fig. 



235 U, nb). 



The cause of the formation of the curvatures, 



which are of fundamental importance in the 



anatomy of the brain, is to be sought princi- 

 pally in the more vigorous longitudinal growth 

 which distinguishes the cerebral tube, and more 

 especially its dorsal wall, from the surrounding 

 parts. As His has established by means of 

 measurements, the fundament of the brain more 

 than doubles its length, while the spinal cord 

 increases by only about one-sixth of its length. 



The cephalic flexure (fig. 235 hh) is developed first. The floor of 

 the fore-brain sinks downward a little around the anterior end of the 

 chorda dorsalis (fig, 237 ch), and forms at first a right angle with 



Fig. 236. — Brain of a 

 human embryo eeven 

 weeks old, parietal 

 (Scheitel) aspect, after 



M.HALKOVICS. 



msp, -Longitudinal or in- 

 terpallial fissure (Man- 

 telspalte), at the bottom 

 of which is seen the 

 embryonic lamina ter- 

 minalis (Schlussplatte) ; 

 Ams, left hemisphere ; 

 2/t, between-brain ; hiA, 

 mid-brain ; hh, hind- 

 brain and after-brain. 



