414 THE PHYSIOLOGY OF REPRODUCTION 



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

 separated by the coelom. The outer layer becomes the false amnion 

 or chorion,^ 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.^ 



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 

 splanchnopleur.^ In some Mammals the cavit^y of the allantois is not 

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

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

 the Primates, however, it projects free for a time into the coelom, 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, f ooti^ote ^ and p. 490. 



2 That the liquor amnii supplies water to the foetus, according to Teldman, 

 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, 

 but contains slight quantities of albumen, fats, and inorganic substances. 

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

 ducts of excretion by the fcetal kidney ; moreover, .];)enzoic acid administered to 

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

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

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

 in the foetal 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 

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

 in the stratified epithi^ium on the inner surface of the amnion, at the Same time 

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

 Post- Natal Child Physiology, London, 1920. . ^ 



3 Morph61ogically, therefore, the allantois is an extra-embryonic bladder. 

 The fluid has begn 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. eit. 



