596 TEXT-BOOK OF EMBRYOLOGY. 



from mother to foetus or from foetus to mother. This can be absolutely proved in 

 early pregnancy by the fact that nucleated red cells are at this stage constantly 

 present in the blood of the fcetiis but never normally present in the maternal 

 circulation. The normal circulation of blood through spaces unlined by endo- 

 thelium is such a remarkable exception in histology that repeated attempts 

 have been made to demonstrate an endothelial lining to the intervillous spaces 

 but, up to the present time, no such lining has been found. 



The manner in which the intervillous spaces are formed still remains the 

 subject of much controversy. The similarity of development in the human 

 ovum and in the ovum of the bat has already been noted. In the bat the 

 chorion when first formed consists of two thin layers, an inner mesodermal 

 layer and an outer ectodermal layer (trophoderm). From analogy there is 

 every reason to believe that the early human chorion has the same struc- 

 ture. Proof of this is, however, as yet wanting, as in the earliest human ova 

 the trophoderm is already a thick layer. There are also present over the 

 entire surface of the chorion and thus in contact not only with the future 

 decidua basalis but also in contact with the entire future decidua capsularis, 

 well developed villi, each consisting of a core of mesoderm and of a thick covering 

 of trophoderm (Fig. 74). Between the villi, bounded by the villi and by the 

 decidua, are ; pools of maternal blood. Peters suggested that rapid prolifera- 

 tion of the cells of the trophoderm might result in an opening up of the maternal 

 vessels with which they came in contact and give rise to repeated effusions of 

 maternal blood. This blood would be poured out mainly within the tropho- 

 derm but bounded externally by the decidua. The blood pools thus formed 

 would represent the first stage in the formation of the intervillous spaces. Ac- 

 cording to Bonnet and others the chorionic villi of the developing placenta are 

 constantly opening up new decidual vessels, the trophoderm eroding or dis- 

 solving more and more decidual tissue, so that the intervillous spaces are con- 

 stantly increasing in size with growth of the placenta. 



The placenta at birth is a discoid mass of tissue between 15 and 20 cm. in 

 diameter, about 3 to 4 cm. thick and weighs from 500 to 1200 grms. As its 

 area of attachment marks the point where the ovum becomes fixed to the 

 uterine mucosa and as the point of fixation of the ovum varies, the placenta may 

 be attached to any portion of the uterine wall. It is most frequently attached 

 in the region of the fundus and more frequently to the posterior wall than to 

 the anterior. If the fixation of the ovum is sufficiently low, the placenta may 

 partly or completely close the internal os, thus giving rise to what is known as 

 placenta pr&via. 



The Umbilical Cord. As the amnion grows and extends ventrally with 

 the ventral bending of the embryonic disk, the yolk stalk and sac, now very 

 much attenuated, become pressed against the cord of mesodermal tissue which 



