968 FORMATIONS FROM THE EPIBLAST. 



form a linear union. A tube is thus formed from the furrow, the medul- 

 lary canal (Fig. 376, II, III). The cells lying next the lumen of the 

 canal become the ciliated cylindrical epithelium of the central canal of 

 the spinal cord; the remaining cells produce the ganglia of the central 

 nervous system and their processes. At the cephalic portion, the 

 medullary canal widens out into the following dilatations, of progres- 

 sively diminishing size: the forebrain, prosencephalon (rudiment of the 

 cerebrum; the midbrain, mesencephalon (quadrigeminate bodies); the 

 hindbrain, metencephalon (cerebellum) ; and the afterbrain, myelen- 

 cephalon (oblongata) (Fig. 376. IV and V; Fig. 374, F\ Fig. 377), which 

 gradually passes over into the spinal cord. Below the hindbrain, 

 in the vicinity of the afterbrain, the spinal furrow does not close, and 

 there remains here an open passage-way to the contiguous lower portion 



onH^ W h! 7 of the Brain of a Human Embryo (after His). V, Primary forebrain vesicle, v', sec- 

 ondary forebrain or hemisphere vesicle; Z, mterbrain vesicle; M, midbrain vesicle; H, hindbrain vesicle; 

 crkm-afflexurelanterio?) * W medullary canal; Nk, nuchal flexure; Bk, pontal flexure; Sk, 



of the fourth ventricle (calamus scriptorius). At the caudal extremity 

 there appears also a dilatation of the medullary canal, the lumbar en- 

 largement. In birds the spinal furrow remains permanently open in 

 this situation, and forms the rhomboid sinus. 



While the medullary canal develops in this way, the primitive 

 streak gradually atrophies, and finally disappears entirely (Fig. 374, F). 

 The medullary canal does not continue in a straight course, but it bends 

 in several places ; namely at the junction of the spinal cord and oblon- 

 gata (nuchal flexure) ; at the juncture of the afterbrain and the hind- 

 brain (pontal flexure) ; finally almost at a right angle between the mid- 

 brain and the forebrain (parietal flexure). At first all of the brain- 

 vesicles are without sulci or gyri. From the forebrain vesicle there 

 grows on each side a pedunculated hollow vesicle (Fig. 376, VI, IX) 

 the primary optic vesicle. The entire remaining portion of the epiblast 



