THE F(ETAL MEMBRANES OF MAMMALS. 227 



before us in the surface-view in figure 130. It is ovate, and shows 

 the primitive streak (pr) in the posterior half, and in front of it a 

 deep dorsal furrow (rj) ; tne extra-einbryonic part of the middle 

 germ-layer can be designated as the vascular area (o), since the first 

 traces of the formation of the vessels and the blood are noticeable 

 in it. 



In the much further developed embryo figured in diagram 2 (at 

 about the ninth day in the Rabbit) the middle germ-layer has spread 

 out over about the third part of the blastula, and now encloses an 

 easily distinguishable body-cavity, since the* parietal and visceral 

 middle layers have separated from each other in the embryonic 

 as well as extra-embryonic regions. It extends as far as the place 

 marked st, where the sinus terminalis is found as the outer limit of 

 the now clearly defined vascular area. 



The embryonic fundament is in the act of being constricted off from 

 the blastodermic vesicle. The head- and tail-ends of the embryo, by 

 foldings of the separate layers, have been elevated from the area 

 pellucida in the same way as in the Chick. As there, a cephalic 

 and pelvic part of the intestinal tract (fore and hind gut) have 

 arisen, witb an anterior and posterior intestinal portal, which open 

 toward the cavity of the blastodermic vesicle. 



At the same time occurs the development of the amnion, which 

 was first recognised in the Mammalia by BAER and BISCHOFF. On the 

 diagrammatic section one sees that the extra-embryonic body-cavity 

 has become very capacious, in that the outer germ-layer with the 

 closely applied parietal middle layer has risen up in the vicinity of 

 the embryo and formed itself into the folds ks and ss. The anterior 

 fold of the amnion (ks) has bent over the head, and the posterior 

 fold (ss) over the tail. The two sheaths lie so close to the embryo 

 in the Mammalia, that in looking from the surface they are not 

 easily recognised, especially as they are extraordinarily transparent. 

 On the third diagram the amniotic folds have greatly enlarged, and 

 have grown toward each other over the back of the embryo till their 



of the allantoi^ has diminished and the yolk-sac has become very small, but the amniotic 

 cavity is in the act of increasing. 



d, Vitelline membrane (zona pellucida) ; d', villi of the same ; sh, serous membrane [serosa] ; 

 ch, chorion ; ch.z, villi of the chorion ; am, amnion ; ks, ss, cephalic and caudal folds of the 

 amnion ; a, outer germ-layer ; a', the same in the extra-embryonic region of the blastula ; 

 m, middle germ layer ; m', the same in the extra-embryonic region ; dd, inner germ-layer ; 

 i, the same in the extra-embryonic region ; df, vascular area ; st, sinus terminalis ; kh, cavity 

 of the blastula, which later on becomes the cavity of the yolk-sac (ds) ; dg, stalk of the yolk- 

 sac (vitelline duct) ; al, allantois ; e, embryo ; r, space between chorion and amnion. extra- 

 embryonic part of the body-cavity, filled with albuminous fluid ; vl, ventral body- wall ; 

 hh, pericardial cavity. 



