314 THE EVOLUTION OF MAN. 



water-membrane, or amnion (Fig. 94, am). The embryo 

 swims in a watery fluid, which fills the space between it 

 and the amnion, and is called the amnion-water, or germ- 

 water (Fig. 94, 4, 5 ah) We shall return hereafter to the 

 significance of this remarkable formation. It is of no 

 interest to us at present, because it bears no direct relation 

 to the formation of the body. 



Among the various appendages, the significance of which 

 we shall presently recognize, we will mention, in passing, the 

 allantois and the yelk-sac. The allantois, or urinary sac 

 (Fig. 94, 3, 4 al), is a pear-shaped bladder, which grows out 

 from the hindmost part of the intestinal canal : the inner- 

 most portion of it afterwards changes into the urinary 

 bladder; the outer part, with its vessels, forms the founda- 

 tion of the placenta. In front of the allantois, the yelk-sac, 

 or navel vesicle (Fig. 94, 3, 4 ds), the remnant of the 

 original intestinal germ-vesicle (Fig. 94, ^ Idi), protrudes from 

 the open abdomen of the embryo (Fig. 94, 3, ^ ds). In a 

 later stage of development of the embryo, in which the intes- 

 tinal and ventral walls are nearly closed, this hangs out 

 from the navel-opening in the form of a little stalked 

 bladder (Fig. 94, 4, 5 ds). Its wall consists of two layers, 

 the inner of which is the intestinal-glandular layer, the 

 outer the intestinal-fibrous layer. It is, therefore, a direct 

 continuation of the intestinal wall. In proportion as the 

 embryo grows larger, this yelk-sac becomes smaller. At 

 first the embryo looks merely like a small appendage on 

 the large intestinal germ- vesicle. But, on the contrary, at 

 a later period, the yelk-sac, or the remnant of the intestinal 

 germ-vesicle, looks like a little purse-shaped appendage of 

 the embryo (Fig. 70). Finally, it loses all importance. The 



