PHYLUM CHORDATA 



557 



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 which arise 

 from the blastoderm around the embryo, and grow upwards and 

 inwards, eventually meeting in the middle over the body of the 

 embryo, and uniting in such a way as to form two layers. Of the 



FIG. 114s. Embryo Rabbit, of about nine days, from the dorsal side. al>, optic vesicle ; 

 in. fold of amnion ; op. area pellucida ; h. heart ; h', medullary plate in the region of the 

 future fore-brain ; h", medullary plate in the region of the future mid-brain ; hh, and h'", 

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

 lary groove ; stz, vertebral zone ; ro, vitelline vein. (From Balfour, after Kolliker.) 



two layers thus formed the outer (Fig. 1149, 2 and 3) is, from its 

 mode of formation, continuous with the trophoblast which invests 

 the yolk-sac and its stalk, and with it forms a complete invest- 

 ment for the entire ovum : in the account of the development 

 of the Bird it has been referred to as the serous membrane. The 

 inner layer or amnion, as in the Bird, forms the wall of a cavity 

 the amniotic cavity (4 and 5, ah) which becomes tensely filled 

 with fluid (the liquor amnii) over the body of the embryo ; this 

 serves the purpose of protecting the delicate embryo from 



