THE PLACENTA. 



59 



by a layer of cellular trophoblast, Langhan's layer, which lies next the mesoderm, 

 and a layer of plasmodium external to the cellular layer. The proximal end 

 of each villus is continuous with the chorion plate of the intervillous spaces, 

 formed by the chorion, and the distal extremity is connected with the plasmodial 

 basal layer of the trophoblast, which forms the outer boundary of the intervillous 

 spaces and which is fused with the maternal decidual tissue. 



After a time branches are projected from the sides of the secondary villi 

 into the intervillous spaces. In this way two sets of secondary villi are 

 differentiated, (1) the anchoring villi (Fig. 79), which cross from the chorion to the 



Intervillous space 



Yolk-sac 



Secondary villus 



Anchoring villus 



Maternal 

 artery 



Umbilical cord- 



Decidu 



Temporari 



herniated sim.... _ mf^-m . 



intestine 



Decidua capsulari 



Trophoblast ot&M 

 chorion Iseve 



Pancreas^ 



Uterine tube 

 Unchanged part of 

 uterine gland 

 Dilated part of 

 uterine gland 



Decidua capsularis 

 ~* Trophoblast 



Mesoderm lining 

 of chorion Iseve 



Mesoderm of amnion 



Ectoderm of amnion 



Amnion cavity 



ium (heart not shown) 



FIG. 77. SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION OF THE UMBILICAL CORD. 

 Note that the expanding amnion has almost obliterated the extra-embryonic coalom which lies between 

 it and the chorion. 



basal layer of trophoblast and are attached to the latter by cell columns, which are 

 the remains of the primary villi which have not been penetrated by the foetal 

 mesoderm, and (2) free or absorbing villi (Fig. 76), which extend from the sides of 

 the original secondary villi into the blood, in the intervillous spaces. 



Whilst the trophoblasfcic invasion of the compact layer of the decidua is 

 proceeding, not only are the interglandular elements of the decidua destroyed, but 

 the walls of the glands also, and, as a consequence, some of the glands in the 

 decidua basalis open for a time into the intervillous spaces, and become filled with 

 blood which passes from the spaces into the gland cavities. In many cases, 

 however, before the glands are destroyed their walls are converted into solid 

 strands of cells, and thus the cavities of their more external undestroyed portions 

 are converted into closed spaces. 



In the early stages the trophoblast is differentiated in a similar manner over 



