THE PLACENTA. 



Decichia basalts 



Blood-vessels 

 Muscular wall 

 of uterus 



Uterine 

 tube 



Trophoblast 



Inner mass of cells 

 Unchanged layer 



Dilated part of gland 

 Inner part of 

 gland 



tortuous, and dilatations form in their walls a short distance from their outer closed 

 extremities. At the same time the interglandular tissue increases in amount, and 

 as a result of the various processes the decidua is thicker, softer, more spongy, and 

 more vascular than the mucous membrane from which it was evolved. 



Partly on account of the dilatation of the deep part of the glands and partly 

 on account of differences in texture of the internal as contrasted with the external 

 part of the decidua, the membrane may be looked upon as consisting of three 

 layers. (1) An internal layer, next the cavity, the stratum compactum. (2) An 

 intermediate layer, the stratum spongiosum, formed largely by the dilated parts 

 of the glands. (3) An external layer, the unchanged layer, in which lie the com- 

 paratively unaltered outer ends of the glands. 



When the zygote, in the morula stage, reaches the uterus, from the tenth to 

 the fourteenth day, it acts as a parasite, it eats its way through the epithelium on 

 the surface of the decidua, and implants itself in the stratum compactum. 



The zygote may penetrate the decidua at any point of the wall of the uterine 

 cavity, but it usually 

 enters at some point of 

 the dorsal or the ventral 

 wall. The entrance gener- 

 ally takes place between 

 the mouths of adjacent 

 glands, which are pushed 

 aside, and the zygote be- 

 comes at once surrounded 

 by the interglandular 

 tissue of the stratum com- 

 pactum of the decidua. 

 The aperture through 

 which it passes may be 

 closed by a nbrinous plug 

 or its margins may con- 

 verge rapidly and fuse 

 together. 



The portion of the de- 

 cidua in which the zygote 

 is embedded is thicker 

 than the other parts of the 

 membrane, and it is separ- 

 ated by the zygote into an 

 internal part, the decidua 

 capsularis, and an external 

 part, the decidua basalis. 

 The junction of the decidua 

 capsularis with the decidua 



basalis is the decidua marginalis, and the remainder of the decidua, by far the larger 

 portion, is the decidua vera. 



As soon as the zygote becomes embedded in the decidua its trophoblast under- 

 goes rapid proliferation. The superficial part of the growing trophoblast becomes 

 converted into a mass of nucleated protoplasm, the plasmodial or syncytial layer, 

 but the inner part remains more or less distinctly cellular. 



The plasmodial portion of the trophoblast invades and destroys the surrounding 

 maternal tissue, and at the same time spaces appear in its substance. As the 

 plasmodium destroys the walls of the dilated maternal blood-vessels, channels are 

 made through which the maternal blood flows into the spaces in the plasmodium, 

 and thus maternal blood begins to circulate in the trophoblast of the zygote. 



In the meantime the extra-embryonic ccelom has appeared in the primary 

 mesoderm of the zygote, and the outer layer of the mesoderm has associated itself 

 with the trophoblast to form the chorion. 



The spaces in the plasmodium enlarge rapidly after the maternal blood 



Cavity 

 cervix uter 



74. SCHEMA OF A FRONTAL SECTION OP THE UTERUS, showing 



the various parts of the decidua and a zygote embedded in the 

 decidua. 



