390 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



quantity has been found in the foetus in cases where the liver has 

 been absent. 



The distinct formation of bile is an indication that the products 

 of foetal metabolism are no longer wholly carried off by the mater- 

 nal circulation ; and to the excretory function of the liver there 

 are now added those of the skin and kidney. Since in man, and 

 in many other animals, such substances as are secreted by the 

 kidney find their way at an early date into the cavity of the 

 amnion, the determination of the history of the renal secretion 

 is a matter of difficulty, for as we have seen the amniotic fluid 

 is derived in part at least directly from the mother, and sub- 

 stances present in it may or may not have been discharged 

 into it by the foetus. The amniotic fluid varies not only in 

 quantity but also in specific gravity (1'002 to 1*086) and in 

 composition, and there does not seem to be any definite relation 

 between its specific gravity and the quantity in which it occurs, 

 or between its specific gravity and the size or age of the foetus. 

 It may be said to contain on the average about 1*6 p.c. of solid 

 matter, of which about '2 are proteids, -8 extractives and -6 

 salts. The proteids are serum albumin and probably para- 

 globulin, mucin or a mucin-like body being also present. Sugar 

 appears to be sometimes present, sometimes absent. The most 

 important constituent is perhaps urea, which seems to be always 

 present. Since this is found at quite an early stage, before any 

 secretion from the foetal kidney could take place, it may be thus 

 considered as derived from the mother and comparable in origin to 

 the urea found in serous fluids ; but since urine containing urea is 

 found in the foetal bladder at least as early as the seventh month, 

 we may conclude that during the later stages of pregnancy, and 

 possibly much earlier, part of the urea of the amniotic fluid comes 

 from the foetal kidney. In some animals, ex. gr. ruminants, the 

 cavity of the allantois remains for a long time permanent and 

 filled with fluid, instead of as in man becoming at an early date 

 obliterated in its distal portion. In these animals the kidneys 

 discharge their secretion into this allantoic sac, and in the contents 

 of the sac is found the body allied to urea, allantoin, so called from 

 its having been first discovered in this situation. Traces of allantoin 

 have als*) been found in human amniotic fluid, which result suggests 

 that this substance is at an early stage formed by the kidney but 

 subsequently gives place to the permanent urea. 



There is no evidence that any sweat is secreted by the foetus 

 in the uterus ; and indeed if any such secretion does take place 

 this can only be for the discharge of solid matter, and not as in 

 the adult for the discharge of water ; but the epidermic scales are 

 undoubtedly shed, arid may be detected in the amniotic fluid. 



962. About the middle of intra-uterine life, when the foetal 

 circulation is in full development, the blood flowing along the um- 

 bilical vein (see Fig. 194) is chiefly carried by the ductus venosus 



