260 



ANATOMY OF VERTEBRATES. 



partially reflected from the ^ vascular ' and ' mucous layers.' ^ The 

 mucous layer is concerned in the formation of the intestinal 

 canal ; and beyond this part, which is at first an open groove, the 

 mucous layer expands over the yolk, which it ultimately incloses, 

 the margins of the ^ vitellicle ' so formed, fig. 134, c, contracting 

 and uniting at the side opposite to the embryo at a sort of ^ ci- 

 catrix, to which the last part of the slime adheres.' ^ The vitellicle 

 is richly vascular, and the surface next the yolk is augmented by 

 ruga3, the yolk in contact with which becomes more liquid, and 

 loses its coagulability. 



At about the fortieth hour in the Common Fowl the limbs begin 

 to bud forth, and a vesicle to protrude from near the anal end of 



the intestine which, rapidly 

 expanding, fig. 1 34, 6, spreads 

 over the embryo, acquiring 

 a close adhesion to the am- 

 nios, ib. a, but remaining 

 distinct from the -vdtellicle, 

 ib. e, c, over which it spreads, 

 finally inclosing the albu- 

 men, and interposing itself 

 between the latter and the 

 lining membrane of the shell. 

 Bloodvessels called ' umbili- 

 cal,' fig. 134, i, are coextend- 

 ' called " allantois," from its con- 

 But that it '^ answers other important purposes, 

 must appear e\ddent from its extent being far beyond what would 

 answer that purpose. I conceive that the side of the bag which 

 surrounds and is in contact with the albumen, acts as the chorion 

 or placenta, for it must be by this surface that the albumen is 

 absorbed and the chick supported. The external part of the bag, 

 which comes in contact T\dth the shell, I conceive to act as lungs, 

 for it is the only part that comes in contact with the air : and on 

 opening an egg with the chick pretty far advanced I find that the 

 blood in the veins is scarlet, while it is of the Modena colour in 

 the arteries of the bag.'"* SchAvann's experiments show that the 

 developement of the chick may go on without oxygen to the 

 fifteenth hour, and that the life of the germ is not destroyed till be- 

 tween the twenty-fourth and thirtieth hour, but that the presence 

 of oxygen is essential to further developement.'^ As the embryo 



Membranes of the Chick third day. LV. 



ed with this bag, which Hunter 



>3 



tammg virme. 



' The mucous layer,/, is shown reflected from the vascular area, g. 



2 XX. vol. V. p. xxi. ^ XX. p. xxiv. * Ib. ^ lxix*. 



