FOETAL NUTRITION: THE PLACENTA 487 



which is crossed by branches of the allantoic vessels to reach an 

 epithelial ridge of cells., 



Cheieopteka. — The mode of embedding in the bat is centric, and 

 the allantoic placenta is discoid. Before segmentation is completed, 

 the fertilised ovum reaches the uterus and invariably enters the right 

 cornu (Ercolani ^). The zona pellucida is already thinned and soon 

 disappears, the spherical blastodermic vesicle lying free in the uterine 

 cavity. 



At the beginning of gestation, according to van Beneden,^ the 

 mucosa is composed of a richly cellular connective tissue, covered by 

 a non-cihated epithelium. Of the glands some are simple tubes, and 

 others divide dichotomously. None open on the mesometrial aspect 

 where the blastocyst later becomes fixed. There also the cellular 

 tissue is not so thick. 



Before fixation of the blastocyst, important changes occur in the 

 mucosa. The sub-epithelial connective tissue cells proliferate and 

 form a distinct oorhpact zone. All the capillaries dilate, even before 

 the disappearance of the zona pellucida, and give off many new 

 branches. The tissue fluids are increased, and a serous fluid is 

 transuded and forms, with the glandular secretion, a coagulum around 

 the ovum (Van der Stricht ^). 



On the mesometrial side, the trophoblast thickens around the 

 formative cell mass, and absorbs the surface epithelium. At the 

 opposite pole the cells are flattened, and they also disappear. 

 The foetal ectoderm, which thus comes in contact with the con- 

 nective tissue, is composed of two layers at the embryonic pole, 

 the' plasmodiblast, and, internally to it, the cytoUast. At the non- 

 embryonic or anti-mesometrial pole the plasmodiblast is absent. 



The decidua also differs at the two poles. Opposite the non- 

 embryonic pole the cells remain epithelioid and undergo little change. 

 Where they come in contact with the trophoblast, they show a 

 tendency to necrose. At the placental pole the deeper layers are 

 also composed of epithelioid cells, but superficially the capillaries 

 continue to dilate and make the layer spongy. The cells between 

 them are in active division, but next the plasmodiblast they 

 degenerate. This layer forms the couche paraplacentaire of Nolf* 

 (Fig. 146). At the placental margin it thins out and disappears. 



1 Ercolani, " Nuove ricerche suUa placenta nei pesci cartilaginosi e nei mam- 

 miferi," Mem, delV Accad. d. Sc. deW histitut. di Bologna, vol. x., 1879. 



^ V. Beneden, " De la formation et de la constitution du placenta chez le 

 niurin," Co'mp. Rend, de la Soc. de Biol., vol. v., 1888. 



3 Van der Strioht, " La fixation de I'oeuf du chauve-souris a I'int^rieur de 

 I'ut^rus," Verh. d. anat. OeselL, 13 Vers., Tiibingen, 1899. 



* Nolf, " Etude des modifications de la muqueuse ut&ine pendant la gestation 

 chez le murin," Arch, de Biol., vol. xiv., 1896. 



