788 HANDBOOK OF PHYSIOLOGY. 



by the cavernous spaces before referred to. Jn this way it is cast off 

 with the foetal membrane and the decidua vera and reflexa, together 

 with a part of the decidua serotina. The remaining portion withers, 

 and disappears by being gradually either absorbed, or thrown off in the 

 uterine discharges or the lochia, which occur at this period. 



A new mucous membrane is of course gradually developed, as the 

 old one, by its transformation into the decidua, ceases to perform its 

 original functions. 



The umbilical cord, which in the latter part of foetal life is almost 

 solely composed of the two arteries and the single vein which respectively 

 convey foetal blood to and from the placenta, contains the remnants of 

 other structures which in the early stages of the development of the 

 embryo were, as already related, of great comparative importance. Thus, 

 in early foetal life, it is composed of the following parts : (1. ) Externally, 

 a layer of the amnion, reflected over it from the umbilicus. (2) The um- 

 bilical vesicle with its duct and appertaining omphalo-mesenteric blood 

 vessels. (3.) The remains of the allantois, and continuous with it the 

 urachus. (4.) The umbilical vessels, which, as just remarked, ultimately 

 form the greater part of the cord. 



THE DEVELOPMENT OF THE ORGANS. 



Before considering very briefly* the main points in the development 

 of the chief organs and tissues of the body, it will be useful to have- 

 before us the following table, compiled by Schafer,f showing the differ- 

 ent parts derived from the three blastodermic layers: 



From the Epiblast. The whole of the nervous system, including 

 not only the central organs (brain and spinal cord), but also the peri- 

 pheral nerves and sympathetic. 



The epithelial structures of the organs of special sense. 



The epidermis and its appendages, including the hair and nails. 



The epithelium of all the glands opening upon the surface of the 

 skin, including the mammary glands, the sweat glands and the sebaceous 

 glands. The muscular fibres of the sweat glands. 



The epithelium of the mouth (except that covering the tongue, and 

 the adjacent posterior part of the floor of the mouth, which is derived 

 from the hypoblast), and that of the glands opening into it. 



The enamel of the teeth. 



The epithelium of the nasal passages, of the adjacent upper part of the 

 pharynx and of all the cavities and glands opening into the nasal pas- 



* For a more detailed account the reader is referred to special text-books of 

 embryology. 



f Quain's Anatomy, Xth Ed., Vol. I., Part I., p. 25. 



