MAMMALIA. 233 



We may commence with a blastodermic vesicle, closely 

 invested by the delicate remnant of the zona radiata, at the 

 stage in which the medullary groove is already established. 

 Around the embryonic area a layer of mesoblast would have 

 extended for a certain distance ; so as to give rise to an area 

 vasculosa, in which however the blood-vessels would not have 

 become definitely established. Such a vesicle is represented 

 diagrammatically in fig. 147, 1. Somewhat later the embryo 

 begins to be folded off, first in front and then behind (fig. 147, 

 2). These folds result in a constriction separating the embryo 

 and the yolk-sack (ds), or as it is known in Mammalian embryo- 

 logy, the umbilical vesicle. The splitting of the mesoblast 

 into a splanchnic and a somatic layer has taken place, and at 

 the front and hind end of the embryo a fold (ks) of the somatic 

 mesoblast and epiblast begins to rise up and grow over the head 

 and tail of the embryo. These two folds form the commence- 

 ment of the amnion. The head and tail folds of the amnion are 

 continued round the two sides of the embryo, till they meet and 

 unite into a continuous fold. This fold grows gradually up- 

 wards, but before it has completely enveloped the embryo, the 

 blood-vessels of the area vasculosa become fully developed. 

 They are arranged in a manner not very different from that in 

 the chick. 



The following is a brief account of their arrangement in the 

 Rabbit : 



The outer boundary of the area, which is continually extending further 

 and further round the umbilical vesicle, is marked by a venous sinus 

 terminalis (fig. 147, st). The area is not, as in the chick, a nearly com- 

 plete circle, but is in front divided by a deep indentation extending inwards 

 to the level of the heart. In consequence of this indentation the sinus 

 terminalis ends in front in two branches, which bend inwards and fall 

 directly into the main vitelline veins. The blood is brought from the 

 dorsal aortas by a series of lateral vitelline arteries, and not by a single 

 pair as in the chick. These arteries break up into a more deeply situated 

 arterial network, from which the blood is continued partly into the sinus 

 terminalis, and partly into a superficial venous network. The hinder end 

 of the heart is continued into two vitelline veins, each of which divides 

 into an anterior and a posterior branch. The anterior branch is a limb 

 of the sinus terminalis, and the posterior and smaller branch is continued 

 towards the hind part of the sinus, near which it ends. On its way it 

 receives, on its outer side, numerous branches from the venous network, 



