62 



HUMAN EMBKYOLOGY. 



chorion plate closing the intervillous spaces internally; (2) the villi; (3) the 

 intervillous spaces ; and (4) the basal layer of the trophoblast, which closes the 

 intervillous spaces externally, and is perforated by the maternal vessels passing to 

 and from the spaces. 



The maternal portion of the completed placenta consists from within outwards 

 of (1) the basal layer of the decidua ; (2) the remains of the spongy layer of the 

 decidua ; and (3) the unchanged layer. 



Placenta 



Spongy layer 



Yolk sacs 



Umbilical cord 



Ectoderm of 

 amnion 



Fused mesoderm of_ 

 amnion and chorion 



Trophoblast 



Spongy layer of decidua . 

 Muscular wall of uterus _ 

 Compact layer of decidua 



Uterine tube 



- Amnion cavity 



-^Spongy layer of 

 decidua 



Fused mesoderm of amnion and chorion 



FIG. 80. SCHEMA OF PREGNANT UTERUS IMMEDIATELY AFTER BIRTH OF THE CHILD, showing commencing 

 separation of the placenta. Part of the umbilical cord is shown in section and part in surface view. The 

 blue streaks in the former part indicate the position occupied by the vitello-intestinal duct in earlier stages. 



The basal layer of the decidua is the remains of the compact part of the decidua 

 basalis of earlier stages. It is fused internally with the basal plate of the tropho- 

 blast, and is continuous externally with the spongy layer. The spongy layer con- 

 sists of a series of cleft-like spaces. These spaces are the compressed remains of 

 the earlier dilated portions of the glands of the stratum spongiosum, from which 

 the epithelial lining has, to a great extent, disappeared. The spongy layer is con- 

 tinuous externally with the unchanged layer, in which lie the unaltered outer 

 parts of the glands and the intervening interglandular tissue. 



