368 THE RABBIT. 



whole development is effected. The yolk-sac of the Mammal, 

 011 the other hand, is a thin-walled vesicle, containing fluid, but 

 110 food matter. 



Hence the causes that led to the formation of a yolk-sac 

 in the bird, i.e. the necessity of constricting off the active 

 from the inactive part of the egg in order to avoid undue 

 distortion of the embryo, will not come into play in the case of 

 the Mammal ; and the formation of a yolk-sac by the rabbit 

 embryo must be explained as due to an inherited tendency, 

 and compels us to infer that Mammals are descended from 

 ancestors which produced large eggs, provided with much food- 

 yolk. Further evidence in support of this view has already been 

 given in the earlier portions of this chapter. 



With regard to the structure of the yolk-sac of the rabbit 

 embryo, it will be seen in Fig. 140 that the wall of the upper 

 portion, rather less than half the entire surface, consists of all 

 three embryonic layers epiblast, mesoblast, and hypoblast 

 excepting only a small patch (Figs. 145, 140, AN'), immediately 

 in front of the head of the embryo, which will be referred to 

 shortly. The wall of the lower half of the yolk-sac contains 

 no mesoblast, but is formed of epiblast and hypoblast alone. 



In the mesoblast of the upper half of the yolk-sac, blood- 

 vessels are present, forming the vitelline circulation, or circula- 

 tion of the vascular area (Fig. 145). The margin of this vascular 

 area, or, what is the same thing, the margin of the mesoblast, 

 is indicated by a circular vessel, the sinus terminalis (Figs. 145, 

 140, 147, 148, si), into which the vitelline artery opens, and 

 from which the blood is distributed over the vascular area 

 before it is returned to the heart by the vitelline veins. 



By the downward projection of the head of the embryo on 

 the tenth day (Fig. 147), the upper wall of the yolk-sac becomes 

 driven ventralwards, and during the succeeding days, as the 

 embryo gets bigger (Fig. 148), this doubling up of the yolk-sac 

 becomes more and more marked. By the thirteenth day the two 

 layers, vascular and non-vascular, are almost in contact with each 

 other, and the cavity of the yolk-sac is practically obliterated. 



The outer or non-vascular wall, which is in contact with the 

 wall of the uterus (cf. Fig. 170, YL), now breaks down and be- 

 comes absorbed. Portions of it persist for a time ; but by 

 about the sixteenth day it has practically disappeared, and the 



