678 HANDBOOK OF PHYSIOLOGY. 



entanglement of foetal villi and maternal sinuses, by means of which the 

 blood of the foetus is enriched and purified after the fashion necessary 

 for the proper growth and development of those parts which it is de- 

 signed to nourish. 



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

 that during the greater portion of intra-nterine 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 

 cavernous 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 dis- 

 charges 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 em- 

 bryo 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, ulti- 

 mately form the greater part of the cord. 



THE DEVELOPMENT OF THE ORGANS. 



It remains now to consider in succession the development of the 

 several organs and systems of organs in the further progress of the em- 

 bryo. The accompanying figure (Fig. 466) shows the chief organs of the 

 body in a moderately early stage of development. 



The Vertebral Column and Cranium The primitive part of 

 the vertebral column in all the Vertebrata is the chorda dor sails (noto- 

 chord), which consists entirely of soft cellular cartilage. This cord 

 tapers to a point at the cranial and caudal extremities of the animal. In 



