FCETAL NUTRITION: THE PLACENTA 443 



nest remaining which formed the placenta. ' After a long interval 

 this was proved to be wrong by Reichert l and Hensen. 2 



The fertilised ovum reaches the uterus as a morula or early 

 blastocyst, surrounded by the zona radiata. On the seventh 

 day the zona disappears and embedding begins, but even before 

 this, according to von Spee, 3 the ovum is fixed by processes 

 which extend from the cells of the implantation pole through 

 the zona and come into direct metabolic relationship with the 

 epithelial cells. As in the mouse, the blastocyst remains small, 



mes 



FIG. 107. Longitudinal section of the uterus and implantation cavity of 

 the guinea-pig. (From Duval's "Le Placenta des Rongeurs,'' Journ. de 

 VAnat. etde la Phys., 1892.) 



mes, mesometrial border ; gl, uterine glands ; Z, uterine lumen ; 

 bl, blastodermic vesicle. 



about one- tenth of a millimetre in diameter. At its point of 

 contact with the mucosa, the epithelium is rapidly eroded, and 

 absorbed along with its fat globules by the foetal ectoderm. At 

 the same time changes occur in the deeper layers. In the non- 

 pregnant uterus two layers are present, a sub-epithelial layer of 

 embryonic connective tissue cells interrupted only by capillaries 

 and glands, and a deeper, more reticulate layer. Before the 

 ninth day of pregnancy, no very marked changes occur in the 



1 Reichert, "Beitrage zur Entwicklungsgeschichte des Meerschweinchens," 

 Abhandl. d. Akad. d. Wissensch. zu Berlin, 1861. 



2 Hensen, " Beobachtungen iiber die Befruchtung und Entwicklung des 

 Kaninchens und Meerschweinchens," Zeit. f. Anat. u. Entwick., vol. i., 1866. 



3 Von Spee, " Die Implantation des Meerschweincheneies in die Uterus- 

 wand," Zeit. f. Morphol. u. Anthropol., vol. hi., 1901. 



