590 



ZOOLOGY 



SECT. 



side of the former. The embryo becomes folded oflf from 

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

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

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

 nected only by a narrow yolk-stalk (Figs. 1218 and 121()): tlie 

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



B 



_fl/» 



Vn,. 1-17. — Embryo Rabbit, of aljout nine days, from Die doxsul side, ah, optic vesicle ; 



«;', fold of iUiiniuH ; i/.s, arc;i opaca ; ap, area iielliicida ; /*, liz, heart ; /(', h", h'", medullary 

 plate in the regions of the future fore-, mid-, and hind-hrain respectively; lili, and kh'", 

 hind-lirain ; i,ili, mid-brain ; ph, pericardial section of body-cavity ; jir, lateral zone ; rf, medul- 

 lary groove ; stz, vertebral zone ; uw, protovertebrje ; r'', anterior part of mescntcron ; vh, 

 fore-brain ; ro, vitelline vein. (From Balfour, after KoUiker.) 



of Yolk. A vascular area early becomes established around the 

 embryo 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 fcetal memhranes. 

 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 somatoplcurc which arise from the blastoderm 



