58 



HUMAN EMBKYOLOGY. 



Intervillous space 

 Maternal blood-vessel 



Spongy layer 

 | Placental area 



if I Unchanged layer of decidua 

 Stratum spongiosum 



Muscular wall of uterus 



Uterine tube 



Secondary villus 

 Amnion cavity 

 Amnion 



Mesodenn linin 

 trophobl 



Trophoblas 



Unchanged part of 

 gland" 



Dilated part of 



gland* 



Cavity of uterus 



Body-stalk 



antoic diverti- 

 um 



Primitive streak 

 JSTeurenteric canal 



Cavity of 

 entoderm sac 



JJxtra-embryonic 

 coelom 

 Decidua capsularis 



Decidua vera 



Embryonic 



begins to circulate within them and the plasmodium becomes divided into 



three series of 

 parts. (1) The 

 parts which lie 

 between adjacent 

 blood spaces, the 

 primary chorionic 

 villi. (2) The parts 

 which lie in con- 

 tac t with the 

 mesoderm of the 

 chorion, and which 

 form with the 

 mesoderm the 

 chorion plate. (3) 

 The parts which 

 cover the maternal 

 tissues and form 

 the outer boun- 

 daries of the blood 

 spaces, the basal 

 layer. The blood 

 spaces themselves 

 are called the in- 

 tervillous spaces 

 (Figs. 76, 79). 



After a time 

 each primary 

 villus differenti- 

 ates into a cellular core and plasmodial periphery, and thereafter the villi are invaded 

 by the mesoderm of 

 the chorion and are 

 thus converted into 

 secondary villi (Fio-. 

 76). 



The first-formed 

 villi are non-vascular, 

 but by the time 'the 

 secondary villi have 

 developed the um- 

 bilical arteries have 

 grown through the 

 body-stalk (allantoic 

 stalk) into the meso- 

 derm of the chorion, 

 and branches from 

 them enter the nieso- 

 dermal cores of the 

 villi, which thus be- 

 come vascular. 



When the second- 

 ary villi are fully 

 developed each con- 

 sists of a vascular 

 mesodermal core con- 

 tinuous with the 

 mesoderm of the 



chorion The meso- FlG- 76 -~ ScHEMA OF A FRONTAL SECTION OF A PREGNANT UTERUS AT THE 



PERIOD OF THE FORMATION OF THE EMBRYO. Note extension of amnion 

 dermal Core IS Covered as contrasted with stage shown in Fig. 75. 



FIG. 75. SCHEMA OF A SECTION OF A PREGNANT UTERUS AFTER THE FORMATION 

 OF THE INTERVILLOUS SPACES. 



Unchanged layer Maternal blood-vessels 



Placental area 

 Spongy layer ^r~ ^* ^. /I Intervillous spaces 



Absorbing 

 chorionic villi 

 Uterine tube 



Trophoblast 

 of chorion 



Amnion 



Amnion w. 

 cavity 



Trophoblast* 



Decidua 

 capsularis 



Decidua vera : 



Spongy layer 

 Extra-em bryoui 



