FCETAL NUTRITION: THE PLACENTA 429 



yet known. In the early stages the blastodermic vesicle is attached 

 to the uterine mucosa by 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; beyond 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 uterus ;^ 

 (2) groups of columnar cells in process of elongation ; and (3) phago- 

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

 and especially in shallow depressions beyond the 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 ). 



Uff 



FIG. 114. Diagram representing a stage in the formation of the placenta of 

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

 Phys., vol. xxxviii., 1904.) 



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

 UE, uterine epithelium ; FE, foetal ectoderm ; FM, foetal mesoderm. 



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

 which are probably lined with maternal epithelium. Between the 

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

 tion. The lymphatic system of the mucosa is enormously developed 

 (Kolster 2 ). 



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

 placenta is found. The former is determined by the presence from 

 an early period, and independently of pregnancy, of numerous 

 prominences or cotyledonary burrs, which project as thickened knobs 

 of the sub-epithelial tissue into the uterine lumen. During preg- 

 nancy they form connections with localised proliferations of the 

 trophoblast. The burrs vary in number from fifty or sixty in the 

 sheep to five or six in the roe-deer. 



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



1 Ewart, Critical Period in the Development of the Horse, London, 1897 ; and 

 i' Studies in the Development of the Horse," Trans. Roy. Soc. Edin., vol. li., 

 1915. 



2 Kolster, " Die Embryotrophe placentarer Sauger, mit besonderer Beriick- 

 sichtigung der Stute," Anat. ffefte, vol. xviii., 1902. 



