FCETAL NUTRITION: THE PLACENTA 439 



present (Jenkinson ^), and the contents of the umbilical vesicle 

 are " not yolk, but another nutritive substance which the 

 ovum, in the absence of yolk, takes from the maternal tissues, 

 viz. haemoglobin " (Sobotta ^). 



The decidual cavity is at first small and ovoid, and has a 

 thick waU. As it grows, the lumen of the uterus is obhterated, 

 and at its point of contact with the mesometrial waU the epi- 

 thehum of the latter disappears. Thereafter the two layers 

 fuse, and at the point of fusion the placenta is developed. The 

 lumen of the uterus is later re-estabhshed, as in the guinea-pig 

 (see Fig. 110), at the floor of the decidual cavity. Hence the 

 primary decidua reflexa forms the serotina, and a secondary 

 reflexa is formed, which is recognisable till the twentieth day 

 of pregnancy. 



The increase in size of the implantation cavity is accom- 

 panied by a thinning of its wall. According to Duval this is a 

 mechanical process, since the cells do not increase in number, 

 but it is probably more comphcated. On the inner surface of 

 the decidua giant-cells appear around the ovum, and they are 

 phagocytic (Fig. 106). Duval stated that each was derived from 

 a cell of the foetal ectodermal wall of the yolk-sac, and later 

 from a ceU of the ectoplacental cone. As they increase in 

 number, they form a distinct layer, two to five cells in depth, 

 between the yolk-sac and the wall of the implantation cavity, 

 and some wander into the decidua and he singly or in groups. 

 In their interior degenerating leucocytes are frequently seen. 

 Sobotta also stated that they were foetal in origin, and helped to 

 fix the ovum and erode maternal capillaries. More recently 

 Kolster has brought forward evidence from their histological 

 appearance that they are transformed decidual cells, and this is 

 strongly supported by Disse's investigations on the field-mouse,' 

 in which the giant-cells are found before the ovum has become 

 embedded, and the first to appear are at an appreciable dis- 

 tance beneath the surface epithelium. A second series of 



' Jenkinson, " Observations on the Histology and Physiology of the 

 Placenta of the Mouse," Tijd. Nederl. Dierk,, Ver. ii., Dl. vii. 



' Sobotta, "Die Entwicklung der Mans," Arch. f. mikr. Anat., vol. Ixi., 

 1903. 



' Disse, "Die Vergrosserung der Eikammer bei der Feldmaus,'' Arch.f, 

 mikr. Anat., vol. Ixvlii., 1906, 



