436 THE PHYSIOLOGY OF REPRODUCTION 



surface and glands. According to Bonnet, it cannot be considered 

 as a fatty degeneration because the cells are otherwise healthy ; it 

 is rather a fatty infiltration, the epithelium secreting it from the 

 lymph or blond-plasma, storing it and later giving it off to the 

 uterine milk. 



Kolster 1 has described a process by means of which cellular 

 elements are added to the " Embryotrophe." 2 The gland epithelium 

 proliferates so strongly that the cells cannot find room in the wall, 

 and tracts of them are invaginated into the lumen. Later the 

 cellular projections, sometimes along with some of the underlying 

 connective tissue as in the mare, are cut off and added to the 

 embryotrophe (Fig. 119). 



Traces of glycogen may be extracted from both the maternal and 

 foetal parts of the cotyledons, but it is too small in amount to be 

 demonstrated histologically. It is also present in small amounts in the 

 extra-cotyledonary areas in the uterine epithelium both superficial 

 and glandular in the cow, in the sub-epithelial connective tissue in 

 the sheep, and in the uterine milk (Jenkinson 3 ). Large quantities 

 of glycogen are stored in the plaques amniotiques, localised masses of 

 cells on the internal surface of the amnion, and later on the umbilical 

 (ord. In the calf embryo the plaques reach their full development 

 about the sixth month, and then gradually atrophy. 



It is obvious that the uterine milk must contain many elements 

 which have not been mentioned individually. The product of 

 conception requires numerous other substances for its development 

 besides protein, fat, carbohydrate, and iron. Organic phosphorus 

 compounds are furnished by the nuclei of cells, and these may also 

 'ontain iron. In general the fixation of mineral elements is slight 

 at the beginning of pregnancy, but becomes active towards the end. 

 I>ut the requirements vary at different periods of pregnancy. For 

 example, sodium decreases and calcium increases with the replace- 

 ment of cartilage by bone, and potassium increases with the 

 increased manufacture of red blood corpuscles. These and many 

 other substances are present in uterine milk though not demonstrated 

 liistologically. Either they have been dissolved by the fixative, or 

 have remained jmstained by the methods hitherto employed. 



One other constituent has been described by various observers, 



1 Kolster, "Die Embryotrophe placentarer .Siiuger, etc.," Aunt. Hefte, vols. 

 xviii. and xix., 1902 and 1903. 



* Objections have been raised to the term " uterine milk " because the fluid 

 contains cellular elements, pigment granules, etc., which are not present in the 

 mammary secretion. Bonnet and his followers have employed the convenient 

 tfi in " Kmbryotrophe," but it must be noted that in the sheep it forms the 

 nutriment long after the embryonic stage of the developing ovum is past. 

 The two terms are used indiscriminately in this chapter. 



3 Jenkinson, "Notes on the Histology and Physiology of the Placenta in 

 Ungulate." Proc. Zool. ,S'oc., London, 1906, vol. i. 



