FCETAL NUTRITION: THE PLACENTA 



429 



yet known. In the early stages the 1)lastodermic vesicle is attached 

 to the nteiine mucosa liy the trophoblast covering the lower pole of 

 the ovum. The trophoblast at the end of the third week differs from 

 that of the sheep and pig and other Ungulates. Up to the sinus 

 terminalis it consists of typical columnar cells ; 1 leyond the sinus 

 there are (1) groups of tall columnar cells arranged to form discs 

 which probably help to fix the blastocyst to the lining of the uteius ; 

 (2) groups of columnar cells in process of elongation ; and (o) phago- 

 cytic columnar cells with sac-like processes placed around the discs 

 and especially in shallow depressions beyond tlie sinus terminalis 

 which probably lie opposite the openings of the uterine glands and 

 are concerned in taking up the more solid particles of uterine milk 

 (Ewart 1). 



UM 



MB 



Fig. 114. — Diagram representing a stage in the foimation of the placenta of 

 the pig. (From Robinson, Hunterian Lectures, Jour, of Anat. and 

 Phi/s., vol. xxxviii., 1904.) 



UM, Uterine muscle ; MB, maternal blood-vessel ; UG, uteiine glands ; 

 UE, uterine epithelium ; FE, fcetal ectoderm ; FM, fietal mesoderm. 



Villi are formed in the allantoic region, and they fit into oypts 

 which are probably lined with maternal epithelium. Between 'the 

 foetal and maternal tissues in the drypt is a space filled with secre- 

 tion. The lymphatic system of the nuicosa is enormously developed 

 (Kolster "). 



Sheep. — In the sheep and cow the poly-cotyledonary type of 

 p)lacenta is found. The foinier is determined by tlie presence from 

 an early period, and independently of pregnancy, of numerous 

 prominences or eotijledonarti lucrrs, which project as thickened knobs 

 of the su))-epithelial tissue into the uterine lumen. Uuring preg- 

 nancy they form connections witli localised proliferations of the 

 trophoblast. The burrs vary in numlier fiom fifty or sixty in the 

 sheep to five or six in tlie roe-deer. 



The ova of the sheep reach the uterus four or five days after 



' Ewart, Cn'timl I'eriod in the Dcreloiniuoit of tin' //orw, London, 1897 ; and 

 "Studies in the Development of the Horse," Tnmx. Hoi/. Sac. Edin., vol. li., 

 1915. 



- Kolstei', " Die Embryotrophe placentarei- Siiuger, mit Ijesonderer Beiiick- 

 sichtigung dei' Stute," Anat. Ifefte, vol. xviii., 1902. 



