38 



CEREBRAL VESICLES. 



number, but subsequently the most anterior (fore-brain) and posterior (hind-brain) 

 each forms two vesicles, whereas the middle vesicle (mid-brain) remains permanently 

 undivided. Five vesicles are therefore then to be seen, and these give rise eventually 



Fig. 40. SURFACE VIEW OF AN EARLY EMBRYO OF THE GUINEA PIG SHOWING THE COMMENCEMENT OF 



THE THREE PRIMARY CEREBRAL VESICLES (1, 2, 3) AS ENLARGEMENTS OF THE MEDULLARY GROOVE. 



Semi-diagrammatic. 



pr, primitive streak and groove. 



Fig. 41. RABBIT EMBRYO OF THE OTH DAY, FROM THE SURFACE. 2 . (Kolliker.) 



The medullary groove is enlarged anteriorly and the primary optic vesicles are growing out from 

 the first cerebral enlargement. On either side of the head, the (double) tubular heart is seen. Eight 

 pairs of protovertebrse are formed. 



to the five fundamental divisions of the brain, while from the sides of the fore-brain 

 the rudiments of the optic nerves and retinas grow out as hollow protrusions. 



Heart and vascular system. While this change is progressing in the 

 neural canal, and the protovertebrse are becoming formed in the paraxial mesoblast, 

 the first sign of a vascular system is beginning to make its appearance in the 

 mesoblast on either side of the head in the form of a simple tubular vessel (fig. 

 41), which becomes developed in the splanchnopleure in this situation. As the 

 splanchnopleure and its accompanying hypoblast fold round on either side under the 

 head to form the fore-gut, these two simple tubes necessarily come together in the 

 middle line, and they then fuse together longitudinally to form a single tube, the 

 primitive heart (fig. 38) ; this tube runs for a short distance in the mesoblast immediately 

 under the fore-gub, and then divides into two branches, which pass laterally, so as to 

 partially encircle the fore-gut, and thence course backwards along the body of the 

 embryo on either side of the notochord. These two vessels form the primitive 

 arteries (primitive aorta), the part of each which encircles the fore-gut as it passes 

 dorsalwards being known as theirs/ aortic arch. On the other hand, the posterior 



