402 THE PHYSIOLOGY OF REPRODUCTION 



But in the placentae of the sloth, the apes, and the human 

 female, where an unusual development of the maternal blood- 

 vessels into larger sinuses takes place, a modification in the 

 anatomical structure is produced which seems to render the 

 presence of such a secretion unnecessary. The nutritive changes 

 in all probability take place directly between the maternal and 

 foetal blood." 



More recent investigations have thrown fresh light on the 

 origin, composition, and absorption of uterine milk. It must 

 be recognised that, even before the onset of pregnancy, changes 

 occur in the uterus which are important for the nourishment 

 of an embryo developing later. Shortly before the first cestrous 

 period, the mucosa " matures " by the formation of the richly 

 cellular sub-epithelial layer of connective tissue already re- 

 ferred to (see p. 398). Among Indeciduates it is specially well 

 marked in the cotyledonary types. With the first procestrum 

 the mucous membrane becomes cedematous, and the super- 

 ficial capillaries are dilated. Many of them rupture and give 

 rise to miliary haemorrhages, which later undergo changes such 

 as occur in haemorrhages elsewhere. Whether the changes are 

 caused by an enzyme action on the part of the leucocytes is 

 uncertain ; but in any case the white corpuscles take up the 

 pigmented products of disintegration, and then arrange them- 

 selves in a row, or in groups, close under the surface epithelium 

 in the manner described in an earlier chapter (Chap. III. p. 109). 

 Some of the leucocytes contain unaltered haemoglobin, as is 

 shown by the characteristic reaction with eosin ; others contain 

 derivatives of it, in which iron may often be demonstrated. 

 With the onset of pregnancy these cells wander out between the 

 epithelial cells, and mingle with the secretion lying in the uterine 

 cavity. This secretion is poured out by the superficial and 

 glandular epithelium, which becomes more active at the be- 

 ginning of pregnancy with the increased flow of blood and 

 lymph through the mucosa. It forms the more fluid part of 

 the uterine milk in which the formed constituents lie. It is 

 necessarily found only in the extra cotyledonary regions since 

 no glands exist in the burrs. 



Besides the intracellular pigments, there is another source 

 of iron, though in widely varying amounts. In all placental 



