484 THE PHYSIOLOGY OF REPRODUCTION 



.]/,,/,.. The method of embedding is centric. A simple yolk-sac 

 placenta exists for a time. The allantoic placenta is discoid and is 

 placed anti-mesometrially. The glandular secretion is of importance 

 for the nourishment of the developing foetus during the greater part 

 of pregnancy (Strahl, 1 Vernhout 2 ). 



At the beginning of pregnancy the mucosa shows variations in its 

 different parts. Near the mesometrium, for about one-third of the 

 circumference of the lumen, the glandular layer is thin. Anti- 

 inesometrially the muscular layer is not so well developed, but 

 superficially to the glands there is a proliferation of connective tissue 

 cells, through which the ducts run to open into the lumen. The first 

 attachment is in this region. 



The uterine horns show a series of small swellings where the ova 

 are present. The blastocysts grow to a comparatively large size, and 

 completely fill up the lumen. By their further growth, the epithelium 

 near the mesometrium is flattened and replaced by trophoblastic cells, 

 which do not penetrate into the connective tissue or form villi. 

 Hence the yolk-sac placenta is of a simple type ; it persists through- 

 out pregnancy. 



On the opposite side the decidual formation proceeds, and the 

 mucosa becomes thicker. In its substance a rich network of blood- 

 capillaries is developed. The epithelial cells lose their boundaries 

 and form a svmplasma. According- to Strahl this remains, and forms 

 the syncytial covering of the future villi, but Vernhout has shown 

 that the trophoblast proliferates and form-; a layer of epithelioid cells 

 which penetrate into the epithelium and absorb and gradually replace 

 it. Over each gland opening the trophoblast forms a dome as in 

 Ruminants (Fig. 144). In the placental region the glandular 

 epithelium is not changed, and around each opening a small area of 

 the surrounding uterine epithelium persists. In the cavity between 

 a gland orifice and its trophoblastic cap lies a dark secretion, pigmented 

 by admixture with extra vasated blood, and the cap is similarly 

 pigmented Hence the secretion is probably absorbed by the foatal 

 ectoderm throughout the greater part of pregnancy during which the 

 glands remain. After the disappearance of the surface epithelium 

 at the point of connection with the blastocyst, the plasmodiblast 

 penetrates into the connective tissue layer which forms a symplasma. 

 It is followed by the cytoblast and the allantoic villi. In the 

 syiicytimn the circulating maternal blood provides for the exchange 

 ises, and supplements the nutriment supplied by the glandular 

 secretion. 



1 Strahl, " Ueber den Ban der Placenta, von 7' ////// i-nropea? Anat. A HZ., vol. v., 

 - \Yrnhout, "Ueber die Placenta des Maulwnrfs," Aunt, ffefte, vol. v., 1894. 



