1132 THE NUTRITION OF THE EMBRYO. [BOOK 



been first discovered in this situation. Traces of allantoin have 

 also been found in human amniotic fluid, which result suggests 

 that this substance is at any 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, and may be detected in the amniotic 

 fluid. 



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

 circulation is in full development, the blood flowing along the 

 umbilical vein (see Fig. 195) is chiefly carried by the ductus 

 venosus into the inferior vena cava and so into the right auricle. 

 Thence it appears to be directed by the valve of Eustachius 

 through the foramen ovale into the left auricle, passing from 

 which into the left ventricle it is driven into the aorta. Part 

 of the umbilical blood, however, instead of passing directly to 

 the inferior cava, enters with the blood carried by the portal 

 vein into the hepatic circulation, from which it returns to the 

 inferior cava by the hepatic veins. The inferior cava also con- 

 tains blood coming from the lower limbs and lower trunk. 

 Hence the blood which passing from the right auricle into the 

 left auricle through the foramen ovale is distributed by the left 

 ventricle through the aortic arch, though chiefly blood coming 

 direct from the placenta, is also blood which on its way from 

 the placenta has passed through the liver and blood derived 

 from the tissues of the lower part of the body of the foetus. 

 The blood descending as foetal venous blood from the head and 

 limbs by the superior vena cava appears not to mingle largely 

 with that of the inferior vena cava, but to fall into the right 

 ventricle, from which it is discharged through the ductus arteri- 

 osus (Botalli) into the aorta below the arch, whence it flows 

 partly to the lower trunk and limbs, but chiefly by the umbili- 

 cal arteries to the placenta. A small quantity only of the con- 

 tents of the right ventricle finds its way into the lungs. Now 

 the blood which comes from the placenta by the umbilical vein 

 direct into the right auricle is, as far as the respiration of the 

 foetus is concerned, arterial blood ; and the portion of umbilical 

 blood which traverses the liver probably loses at this epoch very 

 little oxygen during its transit through that gland, the liver 

 being at this period much more a simple excretory than an 

 actively metabolic organ. Hence the blood of the inferior vena 

 cava, though mixed, is on the whole arterial blood ; and it is 

 this blood which appears to be sent by the left ventricle through 

 the arch of the aorta into the carotid and subclavian arteries. 

 Thus the head of the foetus is provided with blood compara- 

 tively rich in oxygen. The blood descending from the head 





