THE FtETAL MEMBRANES OF MAMMALS. 221 



6. The allantois is evaginated from the ventral wall of the 

 posterior tract of the hind gut (cloaca), grows as a pedunculated sac 

 (1) into the body-cavity, and (2) through the dermal umbilicus into 

 the extra-embryonic part of the same, extends out from here on 

 all sides between the amnion and serosa, and by virtue of its great 

 vascularity functions as an organ of respiration. 



7. At the end of embryonic development the constantly diminish- 

 ing yolk-sac, after the consumption of the yolk, slips through the 

 open dermal umbilicus into the body-cavity, and is employed in the 

 closure of the intestinal umbilicus. 



8. Amnion, serosa, and that part of the allantois which has 

 grown out beyond the embryonic body, are cast off as useless struc- 

 tures at the dermal umbilicus, which becomes closed. 



CHAPTER XII. 

 THE FCETAL MEMBRANES OF MAMMALS. 



IN their early stages of development the fcetal membranes of 

 Mammals present an extraordinary correspondence with those of 

 Reptiles and Birds (fig. 129). We find a yolk-sac (UV) with abun- 

 dant capillaries, an amnion (am), a serous membrane or serosa (sz), 

 and an allantois (ALC) ; we find that, in the same way as before, 

 the embryo is developed out of a small region of the blastula, and is 

 constricted off in the same way from the extra-embryonic area, with 

 which it remains united only by means of a dermal and intestinal 

 yolk-stalk. 



The correspondence becomes a striking one and stimulates to 

 further reflection, when we take into consideration that the develop- 

 mental processes enumerated are primarily evoked by means of the 

 accumulation of yolk-material in the eggs of Reptiles and Birds, and 

 that the eggs of most Mammals lack almost entirely the yolk, are of 

 very small size, undergo total segmentation, and in all these respects 

 resemble more the eggs of Amphioxus. 



Why, then, does the mammalian germ nevertheless undergo 

 metamorphoses which in other cases are only the result of the 

 accumulation of yolk 1 Why is there developed a yolk-sac that 

 contains no yolk, with a system of blood-vessels that it* designed for 

 the resorption of yolk ? 



