270 HAND-BOOK OF PHYSIOLOGY. 



The importance of the placenta is at once apparent if we remember 

 that, during the greater portion of intra-uterine life, the maternal blood 

 circulating in its vessels supplies the foetus with both food and oxygen. 

 It thus performs the functions which in later life are discharged by the 

 alimentary canal and lungs. 



The whole of this structure is not, as might be imagined, thrown off 

 immediately after birth. The greater part, indeed, comes away at that 

 time, as the after-birth; and the separation of this portion takes place by 

 a rending or crushing through of that part at which its cohesion is least 

 strong, namely, where it is most burrowed and undermined by the cav- 

 ernous spaces before referred to. In 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 peculiar transformation into what is called 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.) Exter- 

 nally, a layer of the amnion, reflected over it from the umbilicus. (2.) 

 The umbilical 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. 



DEVELOPMENT OF ORGANS. 



It remains now to consider in succession the development of the several 

 organs and systems of organs in the further progress of the embryo. 

 The accompanying figure (Fig. 429) shows the chief organs of the body 

 in a moderately early stage of development. 



DEVELOPMENT OF THE VERTEBRAL COLUMN AND CRANIUM. 



The primitive part of the vertebral column in all the Vertebrata 

 is the chorda dorsalis (notochord), which consists entirely of soft 

 cellular cartilage. This cord tapers to a point at the cranial and 

 caudal extremities of the animal. In the progress of its develop- 

 ment, it is found to become enclosed in a membranous sheath, which 



