4H THE PHYSIOLOGY OF REPRODUCTION 



(each of which is composed of ectoderm and somatopleur) are 

 separated by the coelorn. The outer layer becomes the false amnioii 

 or chorion, 1 and the inner layer the true amnion. Within the latter 

 and immediately surrounding the embryo is the liquor amnii. This 

 fluid protects the embryo from injuries and changes of temperature 

 besides affording freedom of movement ; moreover, it acts as a 

 dilating wedge during parturition. It supplies water to the embryo 

 but not nourishment. 2 



While the changes above described are taking place, the allantois 

 grows out (on the tenth day in the rabbit) from the hind- gut as a 

 vesicle lined by hypoblast, and covered externally by a layer of 

 splanchiiopleur. 3 In some Mammals the cavity of the allantois is not 

 continued beyond the body-wall of the embryo, the extra- embryonic 

 portion consisting of a solid rod of inesoblast. In all orders below 

 the Primates, however, it projects free for a time into the coalom, and 

 later fuses, except in the Marsupials, with the whole or part of the 

 outer wall of the blastocyst. In the allantoic mesoblast many vessels 

 are developed, and branches extend into the projections which form 

 the cores of the villi. The blood is brought by two allantoic arteries 

 continued from the terminal bifurcation of the dorsal aorta, and 

 returned by one, or more rarely two, allantoic veins. " While the 

 placenta is being developed, the folding off of the embryo from the 

 yolk-sac becomes more complete, and the yolk-sac remains connected 

 with the ileal region of the intestine by a narrow stalk, the vitelline 



1 See p. 476, footnote 3 , and p. 490. 



2 That the liquor amnii supplies water to the fcetus, according to Feldman, 

 is shown by the fact that hair, epidermal scales, etc., have been found amid the 

 intestinal contents of the newly born. On the other hand, a well-nourished 

 foetus may be found with almost complete absence of liquor amnii or with the 

 oesophageal lumen occluded, and so preventing it from swallowing the fluid. 

 The liquor amnii is slightly alkaline in reaction; it consists chiefly of water, 

 b.ut contains slight quantities of albumen, fats, and inorganic substances. 

 Urea and creatinin have been found in it, indicating that it may receive pro- 

 ducts of excretion by the foetal kidney ; moreover, benzoic acid administered to 

 a pregnant animal led to the appearance of hippuric acid in the liquor amnii. 

 As evidence that the fluid is partly of maternal origin are the facts that 

 potassium iodide, etc., given to the mother appears in the liquor amnii but not 

 in the fetal urine, and that sugar is present in the fluid if the mother has 

 diabetes (see below, p. 463). Jenkinson, however ( Vertebrate Embryology, Oxford, 

 1913), describes glucose as present normally in the liquor amnii of the cow, and 

 says that it increases towards the end of pregnancy, while glycogen, stored up 

 in the stratified epithelium on the inner surface of the' amnion, at the .same time 

 diminishes. For further evidence see Feldman, Principles of Ante-Natal and 

 Post- Natal Child Physiology, London, 1920. 



3 Morphologically, therefore, the allantois is an extra-embryonic bladder. 

 The fluid has been found to contain glucose ('3 per cent.), albumen, mucin, 

 magnesium, sodium and calcium phosphates, sodium chloride, sodium sulphate, 

 and crystals of calcium oxalate, besides a yellow pigment, and allantoin, a sub- 

 stance chemically related to uric acid. It seems, therefore, that the allantois, 

 besides its more important function in connection with the formation of the 

 placenta, acts as a receptacle for foetal excretory products. See Jenkinson, loc. cit. 



