422 



EMBRYOLOGY. 



lateral walls of the hind-brain, which from the beginning has been 

 the longest portion, acquire a constriction which divides the hind- 

 brain into two vesicles, that of the cere- 

 bellum (kh) and the medulla (nh), or 

 after-brain. 



The five-fold segmentation of the 

 neural tube (fig. 235) soon succeeds 

 the four-fold condition; by means of 

 it the fore-brain vesicle undergoes 

 fundamental transformations. First, 

 the primary optic vesicles (au) begin 

 to be constricted off from the fore- 

 brain vesicle, until they remain at- 

 tached by only slender, hollow stalks. 

 Since the constriction takes place 

 mainly from above downward, the 

 stalks remain in connection with the 

 base of the fore-brain vesicle. The 

 front wall of the vesicle then begins 

 to protrude anteriorly, and to be 

 marked off by means of a lateral 

 furrow, which runs from above and 

 behind obliquely downward and for- 

 ward. In this manner the primary 

 vesicle of the fore-brain, like the 

 hind -brain vesicle, is secondarily di- 

 vided into two portions, which we 

 can now distinguish as the vesicles 

 of the cerebrum and the between-brain 



(y/i, zh). The optic nerves remain united with the base of the latter.. 

 The vesicle of the cerebrum is distinguished by a very rapid 

 growth, and soon begins to surpass all the other parts of the brain 

 in size. But it becomes divided before this into right and left hah < >. 

 From the connective tissue enveloping the neural tube there grows 

 down in the median plane a process, the future fabc crrebri. This 

 growth advances from above and in front against the cerebral vi- 

 and deeply infolds its upper wall. The halves (fig. 236 /tins) that have 

 tlm- arisen are united at their bnsi s ; t h-y present a more flat median 

 and a convex outer surface, and are called the two vesicles of tit e hemi- 

 spheres, since they furnish the foundation for the cerebral hemispheres. 

 The separate regions of the brain-tube produced by constrictions 



Tig, 234.- Dorsal aspect, by trans- 

 mitted light, of the head of a 

 Chick incubated 58 hours, after 

 MIHALKOVICS. Magnified 40 

 diameters. 



x, Anterior wall of the primary fore- 

 brain vesicle, which afterwards 

 evaginates to form the cerebrum ; 

 pvh, primary fore-brain vesicle ; 

 au, optic vesicle ; mh, mid-brain 

 vesicle ; IcJt, vesicle of the cere- 

 bellum ; nh, after-brain vesicle ; 

 h, heart ; vo, omphalomesenteric 

 vein ; rrn, spinal cord ; us, 

 primitive segment. 



