590 



ZOOLOGY 



side of the former. The embryo becomes folded off from 

 the blastoderm as in the Bird, and at length the body of 

 the young Mammal becomes constricted off from the " yolk-sac " 

 or umUlical vesicle, so that, ultimately, the two come to be con- 

 nected only by a narrow yolk-stalk (Figs. 1218 and 1219) : the 

 yolk-sac is a thin- walled sac containing a coagulable fluid in place 





Fig. 1217. — Embryo Rabbit, of almut nine days, from the dorsal side. o&, optic vesicle ; 

 ft;', fold of amnion ; as, area opaca ; aj>, area pellucida ; h, liz, heart ; h', h", h'", medullary 

 pliite in the regions of the futiu'e fore-, mid-, and hind-brain respectively ; hh, and hli''\ 

 hind-brain ; rah, mid-brain ; 'ph, pericardial section of body-cavity ; pz, lateral zone ; rf, medul - 

 Jaiy groove ; siz, vertebral zone : -uw, protovertebraj ; vd, anterior part of mesenteron ; vh, 

 fore-brain ; vo, vitelline vein. (From Balfour, after Kolliker.) 



of yolk. A vascular area early becomes established around the 

 embiyo on the wall of the yolk-sac. 



The most important of the points of difference between a 

 Mammal and a Bird, as regards the later part of the history of the 

 development, are connected with the fate of the foetal membranes. 

 The amnion is in many Mammals developed in the same way as 

 in the Bird, viz. : by the formation of a system of folds of the 

 extra-embryonal somatopleure which arise from the blastoderm 



