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 

 iDehind 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 

 (gh, 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 halves. 

 From the connective tissue envelojang the neural tube there grows 

 down in the median plane a process, the future falx cerebri. This 

 growth advances from above and in front against the cerebral vesicle 

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

 thus arisen are united at their bases ; they present a more flat median 

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

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

 The separate regions of the brain-tube produced by constrictions 



Fig. 234.- Dorsal aspect, by trans- 

 mitted light, 'of the head of a 

 Chick incubated 56 hours, after 

 MlHALKOVlcs. Magnified 40 

 diameters. 



a:, Anterior wall of the primary fore- 

 brain vesicle, which afterwards 

 evaginates to fonn the cerebrum ; 

 pvhf primary fore-brain vesicle ; 

 au, optic vesicle ; 'inli, mid-brain 

 vesicle ; M, vesicle of the cere- 

 bellum ; nh, after-brain vesicle ; 

 h, heart ; vo, omphalomesenteric 

 vein ; ■ rm, spinal cord ; us, 

 primitive segment. 



