THE HLOOD-VESSELS. 297 



oxygen from the allantois, it follows that the blood entering the 

 heart by the posterior vena cava is arterial, and not venous, in 

 character. The right understanding of the peculiarities in the 

 circulation in the chick during embryonic life is mainly depen- 

 dent on a full appreciation of this fact. 



The aortic arches of the chick embryo undergo changes very 

 similar to those which occur in the frog ; the arches disap- 

 pearing in part, and in part becoming modified into the arterial 

 system of the adult. As in the frog, the pulmonary arteries are 

 branches from the hindmost pair of aortic arches. 



Histological development of the blood-vessels. The blood- 

 vessels appear in the vascular area before they are formed in 

 the embryo itself, and the mode of their development is easier 

 to determine in the former situation. 



Shortly before the end of the first day, when two or three 

 pairs of mesoblastic somites are present in the embryo, a 

 number of outgrowths from the upper surface of the hypoblast 

 appear round the inner margin of the area opaca. These 

 branch freely, and unite with one another to form a network, 

 lying between the mesoblast and the hypoblast : the strands of 

 the network are solid ; they contain numerous nuclei, but cell 

 outlines are difficult or impossible to determine in them. 

 Within the strands, vacuoles soon appear at intervals : these 

 enlarge rapidly, and, running into one another, convert the solid 

 network into a system of anastomosing tubules with nucleated 

 walls. These tubules are capillary blood-vessels ; they are filled 

 with fluid, but contain no blood corpuscles until a later stage. 



This vascular network spreads rapidly, extending outwards 

 as the vascular area widens, and inwards across the area 

 pellucida to the embryo, which it invades on the second day. 

 From their first appearance the vessels of the embryo are con- 

 tinuous with those of the area pellucida ; but it is not quite clear 

 how far they arise in situ, or how far by intrusion of vessels 

 from the area pellucida. 



This network of blood-vessels lies below the mesoblast, be- 

 tween this and the hypoblast ; it is connected at places with the 

 hypoblast, from which it arises in the first instance, but it is 

 quite independent of the mesoblast. If this appears to contradict 

 the general rule according to which the blood-vessels are derived 



