THE ORGANS OF THE OUTER GERM-LAYER. 



423 



and evaginations subsequently become still more sharply marked 

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



kh rf gb nil 



nb 



Fig. 235. Brain of a human embryo of the third week (Ly). Profile reconstruction. After His. 

 gh, Cerebral vesicle ; -/t, between-brain vesicle : mh. mid-brain vesicle; kh, nh, vesicles of the 



cerebellum and medulla oblongata ; OH, optic vesicle ; gb, auditory vesicle ; tr, infundibiilum ; 



'//, 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 constricted 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 cephalic, pontal, and nuchal flexures (fig. 

 235 l-b, 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 kb) 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 seven 

 weeks old, parietal 

 (Scheitel) aspect, after 

 MIHALKOVICS. 



msp, Longitudinal or in- 

 terpallial fissure (Man 

 telspalte), at the bottom 

 of which is seen the 

 embryonic lamina ter- 

 minalis(Schlussplatte) 

 hius, left hemisphere ; 

 zh, between-brain ; mh, 

 mid-brain ; hh, hind- 

 brain and after-brain. 



