i in: id. i \ i. MI.MI:I:.\M-:S OF \i \MM \ -J-l 



6. The allantois i> evaluated fr.un the ventral wall of tin- 

 jiosierit.T tract of the hind ijut i cloaca ), L'I ! pfduncul.t: . -. : 



( 1 ) into the body-cavity, and rJ) throuirh the drrmal uiiil>ilim> into 

 the extra rmhryonic part of the same, extends out from here on 

 all .-Mrs between the aninion and mOSA, and liy virtu-- of its ^n-at 

 va-cularity functions as an oi^an of respiration. 



7. At the end of embryonic development tin- con-t.mtly diminish- 

 ing ydk sac, after the consumption of the yolk, >lips through the 

 o| ten 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 

 i. r rown 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 fu-tal im-mbi-aiios 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 (a/;i), 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 blast ula. and is 

 constricted off in the same way from the extra-embryonic ami, 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- 

 ment al processes enumerated are primarily evoked by means of the 

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

 t hat the eggs of most Mammals lack almost entirely the yolk, are of 

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

 resemble more the e^^ of Amphiozos. 



Why, then, does the mammalian germ nevertheless undergo 

 m< tarnorphoses which in other cases are only the result of the 

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

 contains no yolk, with a sy>tem of blood-vessels tl.at is designed for 

 the ivxjrptioii of yolk? 



