DEVELOPMENT OF THE FCETUS 



361 



ably derived directly from the decidual cells and uterine 

 glands. 



Soon, however, the outermost layer of the embryo 

 becomes speciahsed for the provision of nutrition — ^for 

 this reason it is called the trophoblast. The trophoblast, 



.OEClDUA VERA 



-DECIDUA REFLECTA 



,DE.CIDUA BASAUS 



BLASTODERMIC 

 VESICLE. 



DE.CIOUA VERA 



CERVIX 



DECIDUA VERA 



VAQiNa 



Fig. 69. — Diagrammatic section of the pregmint uterus to show the 

 three parts of tlio decidua (Keith). 



and a layer of mesoblast which surrounds the embryo 

 together form the chorion. This becomes divided into 

 two layers : (a) the Basal or Langhan's layer on the 

 inner side, and (6) the Syncytium, a mass of proto- 

 plasm containing nuclei but no proper cell divisions. 

 The syncytium is powerfully phagocytic. It invades the 

 decidua, eroding not only the decidual cells but also the 

 walls of the capillaries. The maternal blood oozes into the 



