584 DEVELOPMENT. [CHAP. XLI. 



embryo itself taking the form of a boat with its concave surface 

 downwards. This is the first appearance of a visceral cavity, 

 bounded by the ventral laminae, in which the ribs and the trans- 

 verse processes of the vertebrae are formed. 



That part of the most superficial layer of the germinal mem- 

 brane, or serous lamina, immediately surrounding the embryo, 

 forms a prominent fold which soon rises above the surface. Ac- 

 cording to Reichert, however, the fold is formed from the most 

 superficial layer of the membrana intermedia, and is only covered by 

 the investing membrane. Each fold approaches that on the oppo- 

 site side, until, by their approximation and communication, a shut 

 sac is formed, into which fluid is poured. In this fluid, the embryo 

 floats. Its open ventral surface gradually becomes closed, until at 

 last it is connected with the yolk only by means of a very narrow 

 pedicle or cord (umbilical cord), which consists of a narrow tube 

 passing from the intestine to the yolk, with certain vascular trunks, 

 through which the nutritive matter absorbed by the vessels ramify- 

 ing upon the surface of the yolk is carried to the embryo. Of this 

 attenuated cord connecting the embryo with the yolk sac or umbi- 

 lical vesicle we shall speak at some length hereafter. Thus is formed 

 the amnion, to which we shall have to allude more particularly in 

 Chapter XLIII. 



Such are some of the most important changes occurring during 

 the earliest period of incubation in the chick. We shall now con- 

 sider the nature of those which take place in the mammalian 

 ovum. 



CHANGES IN THE MAMMALIAN OVUM. 



The essential changes which manifest themselves in the early 

 period of the development of the mammalian embryo are very 

 similar to those which we have briefly described as occurring in the 

 bird's egg ; but in consequence of the small amount of yolk in the 

 former compared with the latter, and therefore the greater depend- 

 ence of the embryo for its nutrition upon external sources, certain 

 differences are observed in the development of the mammalian 

 ovum. 



It has been already stated (p. 574) that by the time the 

 mammalian ovum has reached the uterus, the process of seg- 

 mentation is complete. It is also paler and more transparent. 

 The germinal vesicle had disappeared previous to segmentation ; 



