536 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



be the ghostly vestiges of formerly existing segments (see pp. 

 210, 211). 



The central nervous system has made important advances in 

 development. The brain shows a relatively large increase in size 

 as compared with the spinal cord : thalamencephaion, mesencephalon 

 and rhombencephalon are marked off by definite constrictions — the 

 mesencephalon being particularly prominent at the bend of the head. 

 The greater part of the roof of the rhombencephalon is assuming its 

 definitive thin membranous character. The three great organs of 

 special sense have made their appearance. The eye (E) forms a 

 large conspicuous cup-like structure lying at the side of the fore- 

 brain. Its rim is cleft ventrally by the choroid fissure (Fig. 236). 

 Its mouth is partially blocked by the round lens rudiment. The 

 otocyst (ot) is also conspicuous — a pear-shaped sac, its narrow end 

 dorsal, lying at the side of the hind brain. The olfactory organ is 

 represented by a slight dimple of thickened ectoderm near the tip of 

 the head. 



The side walls of the foregut are perforated by visceral clefts. 

 The series of these develop from before backwards and by this stage 

 three have commonly appeared — clefts I, II, and III of the series. 



It is perhaps the vascular system which shows the most interest- 

 ing features during the third day. The heart is still in the form of 

 a simple tube, but its active growth in length has caused a great 

 increase in the curvature which was already pronounced about the 

 middle of the second day. It's y-like curvature is shown in Fig. 

 236. At its morphologically front end the heart is continued into 

 the ventral aorta and this at its end gives off a series of vessels, the 

 aortic arches, which pass up round the sides of the foregut between 

 adjacent gill-clefts and open dorsally into the aortic root which lies 

 just dorsal to the clefts. Like the clefts themselves the aortic 

 arches develop in sequence from before backwards and by this stage 

 arches I, II, and III have made their appearance»(Fig. 241, A). 



At its front end the aortic root can be traced for some distance 

 into the head as the dorsal carotid artery (Fig. 241, A, d.c). 

 Posteriorly the two aortic roots become hidden from view by the 

 myotomes but the study of sections shows that they have here united 

 to form the unpaired, dorsal aorta. Still farther back this vessel 

 again becomes paired and a little behind the point of bifurcation 

 each of the branches gives off a large vitelline artery (v. a) which 

 passes outwards at right angles to the axis of the body to supply 

 the vascular area. 



Of the veuous system the most conspicuous components are the 

 great vitelline veins (Fig. 241, A, v.v) which, receiving numerous 

 branches from the vascular area, pass forwards converging towards 

 one another to form by their fusion the hind end of the heart. 

 Examination of the vascular area shows that the branches of the 

 vitelline arteries and of the veins accompany one another in their 

 ramifications. In the living condition, in which all these arrangements 



