CHAP. XI.] MEMBRANES OF BABBIT. 343 



established. Such a vesicle is represented diagram- 

 matically in Fig. 114, i. Somewhat later the embryo 

 begins to be folded off first in front and then behind 

 (Fig. 114, 2). These folds result in a constriction sepa- 

 rating the embryo and the yolk-sac (ds), or as it is 

 called in Mammalian embryology, the v/mbilical 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 meso- 

 blast and epiblast begins to rise up and grow over the 

 head and tail of the embryo. These two folds form the 

 commencement of the amnion. The head and tail folds 

 of the amnion are continued round the two sides of the 

 embryo tUl they meet and unite into a continuous fold. 

 This fold grows gradually upwards, 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 arrange- 

 ment in the rabbit : — 



The outer boundary of the area, which is continually extend- 

 ing further and further round the umbilical Tssicle, ia marked by 

 a venous sinus terminaUs (Pig. 114, st). The area is not, as in 

 the chick, a nearly complete 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 

 aortse 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 



