XIV ELEMENTS OF VERTEBRATE EMBRYOLOGY 461 



to localized thickenings of the splanchnic layer of mesoderm. These 

 thickenings — the blood islands — gradually spread and become joined 

 together into a network. This is the rudiment of a network of blood- 

 vessels which has for its function the absorbing of food-material from 

 the underlying yolk : the portion of opaque area in which the network 

 is present is known as the vascular area. During the conversion of the 

 strands of the at first solid network into blood-vessels the superficial cells 

 of the strand become converted into the wall of the vessel while the 

 enclosed cells become loosened from one another — fluid accumulating 

 between them — and become the blood itself. The network of the vascular 

 area is continued across the pellucid area to the vitelline veins into 

 which the blood drains on its way back to the heart, but in the region 

 of the pellucid area it is much less conspicuous, the network in this region 

 forming only the thin walls of the vessels with fluid contents but without 

 the opaque masses of corpuscles. 



In an egg that has been incubated for nearly three days the embryo 

 has the appearance depicted in Fig. 194. The blastoderm has now 

 spread over about half the surface of the egg while the vascular area has 

 also extended and is now much more conspicuous, the vessels being 

 distended with bright red circulating blood. The yolk is assuming a 

 much more fluid consistency than it had in earlier stages and the albumen 

 is diminishing in amount. A striking change has come over the head 

 region which, owing to its dorsal side growing more actively than its 

 ventral, has become bent ventralwards into a kind of retort shape and, 

 in correlation with this, has become twisted over so as to lie on its left 

 side. Looking down upon the embryo in the opened egg one consequently 

 sees the head region in side view, from its right side, while the portions 

 of the embryo further back are still seen in dorsal view. The main 

 regions of the brain are now quite distinct — the rhombencephalon with 

 its thin membranous roof, the rounded bulging mesencephalon, and the 

 thalamencephalon. In an embryo slightly more advanced than that 

 figured there would be visible the rudiment of the pineal body — a 

 median finger-like projection of the roof of the thalamencephalon — and 

 of the hemispheres — laterally placed bulgings of its wall near its anterior 

 end. 



By this time the amnion has made its appearance. It originates as 

 a fold of the somatopleure which rises up all round the body of the 

 embryo and then grows inwards, overlapping the embryonic body so that 

 the latter becomes gradually covered in as the opening bounded by the 

 rim of the amniotic fold becomes smaller and smaller. In the three-day 

 Fowl embryo the amniotic opening may still be seen (Fig. 194, a.e) and 



