434 THE PHYSIOLOGY OF REPRODUCTION 



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 referred to (see p. 431). Among Indeciduates it is 

 specially well marked in the cotyledonary types. With the first 

 prooestrum the mucous membrane becomes oedematous, an-d the super- 

 ficial capillaries are dilated. Many of them rupture and give rise to 

 miUary hEemorrhages, 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 dis- 

 integration, and then arrange themselves in a row, or in groups, close 

 under the surface epithelium' in the manner described in an earlier 

 chapter (Chapter III.,, p. 105). 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 beginning 

 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 Mammals a 

 greater or less amount of maternal blood is in direct contact with the 

 trophoblast. In the pig and mare it is restricted to individual red 

 blood corpuscles, which find their way to the surface and mingle with 

 the gland secretion. In the ass Strahl ^ has found blood in greater 

 amount, forming small effusions. In the sheep its presence has often 

 been noted by Tafani,^ Bonnet,^ and others. The position of the 

 extravasations in the placenta has been already referred to (see p. 431). 

 In the cow they are apparently not a constant phenomenon, the 

 supply being often restricted, as in the mare, to a few single 

 erythrocytes. In the deer, blood is effused into the glands, but no 

 extravasations take pkce in the cotyledons. Here the whole of the 



1 Strahl, see Hertwig's Handb. d. vergl. u. easp. Entwickelungsg. d. Wirhel- 

 thiere, 1902. 



2 Tafani, " Sulle Condizioni utero-placentari della Vita Fetale," Arch. deUa 

 Scuola d'Anat.-Path., Krenze, 1886. 



3 Bonnet, " Ueber Embryotrophe," Deut. tned. WocL, 1899. 



