60 



HUMAN EMBEYOLOGY. 



the whole of the surface of the zygote, and thus, for a time, the whole of the 

 surface of the chorion is covered with villi. As the embryo grows, and the 

 amnion and the extra- embryonic ccelom are distended, the zygote increases in size, 

 and the capsular portion of the decidua is stretched till its vascular supply is 

 interfered with and the villi associated with it undergo atrophy and disappear. 

 When these degenerative changes have occurred, the portion of the chorion in 

 association with the thinned decidua capsularis presents a relatively smooth 

 surface, and is known as the chorion Iseve. Whilst the decidua capsularis is being 

 stretched and thinned, and the associated portion of the chorion is being reduced 

 to the condition of a non-villous region, the decidua basalis increases in thickness ; 

 at the same time the villi associated with it increase in size and in the complexity 

 of their branches. The portion of the chorion from which these large villi spring 

 is termed the chorion frondosum. It is this portion of the chorion which takes 



Placental area 



Intervillous space 



Spongy layer 



Umbilical 

 cord 



Muscular wall of uterus 

 x'Amnion cavity 



Uterine tube 



Compact layer of decidua 

 - Trophoblast 



Fused mesoderm 

 of chorion and 

 amnion 



Ectoderm of 

 Button 



Spongy layer 



Rectum 

 Small intestine 



, Amnion cavity 



Liver 



Stomacl 



Trachea 



FIG. 78. SCHEMA OP A SECTION OF A PREGNANT UTERUS AFTER FUSION OF AMNION AND CHORION. 



part in the formation of the so-called foetal portion of the placenta, the maternal 

 part of that organ being formed by the decidua basalis. 



The placenta, therefore, is formed partly by the zygote and partly by maternal 

 tissues, but the interchanges between the foetal and the maternal blood take place 

 in the substance of the zygote through the trophoblast which covers the surfaces 

 of the villi. 



As the growth of the embryo and the distension of the amnion continue, 

 the outer surface of the amnion is gradually forced against the inner surface of 

 the chorion, with which it fuses. When this fusion is completed the extra- 

 embryonic ccelom is obliterated and the zygote contains only one extra-embryonic 

 cavity, the amniotic cavity, in which the foetus floats in the amnion fluid (Fig. 78). 



At this period the amnion cavity is bounded by a wall formed by the fused 

 amnion chorion and decidua. In the meantime the chorion has differentiated into 

 the chorion Iseve, fused with the decidua capsularis, and the chorion frondosum, 

 fused with the decidua basilis. As the distension of the amnion proceeds to a 

 still greater extent, the part of the wall of the cavity formed by the fused amnion 

 chorion Iseve and the decidua capsularis projects more and more into the cavity 



