GENERATION AND DEVELOPMENT 



709 



whole surface of the chorion, the placenta is diffuse, as seen in the 

 sow, mare, and camel. The only parts of the chorion in these 

 animals destitute of villi are the poles, and the smooth patch is 

 very minute. If the villi are gathered into tufts upon the surface 

 of the chorion, and these tufts correspond to elevations of the 

 mucous membrane of the uterus, the placenta is cotyledonary or 

 polycotyledonary . The tufts and elevations are the foetal and 

 maternal cotyledons respectively, and number sixty, more or less. 

 If the villi are disposed in a strap-like manner around the envelopes, 

 leaving the poles for some distance free from villi, the placenta is 

 zonary, and such a condition is found in the placenta of the dog and 

 cat. In the rabbit and woman the placenta, from its shape, is 

 discoidal or metadiscoidal. 



Foetal Membranes. — If the egg of the hen be examined after 

 incubating nine days, the appearance seen in Fig. 248 presents 



allasUois 



asnsuons 



airspace. 



shell, 



yolk- sac. 



while. 



Fig. 248. — Hen's Egg at the Ninth Day of Incubation (Ewart after 

 Milnes Marshall). 



itself. A chick in an advanced stage of development is bound 

 within a thin tough skin, containing fluid ; this water-jacket 

 is known as the amnion, and its use is to prevent jar when the 

 egg is moved. An identical arrangement exists in mammals. 

 The supply of food required by the embryo chick during develop- 

 ment is contained in the yolk sac ; to this food-supply the embryo 

 is connected by a stalk through which the nourishment enters 

 its body. The walls of the yolk sac are vascular and connected 

 with the vessels of the embryo. It is through the medium of 

 the vascular wall that the altered yolk is taken up. The modified 

 yolk sac is found in mammals (Figs. 246, 247). It does not 

 contain yolk, but takes up the nourishment secreted by the uterine 

 glands, and for a time this suffices for the needs of the embryo. 

 The chick has another foetal appendage known as the allantois ; 



