1124 THE NUTRITION OF THE EMBRYO. [Book iv. 



synthetic, directed to the building up of the tissues, and such 

 processes consume very little oxygen compared with the pro- 

 cesses leading to expenditure of energy in movement and heat. 

 Hence the simple method of nutrition and respiration by means 

 of the direct contact of the cells of the uterine mucous mem- 

 brane is exchanged for the special vascular mechanism of the 

 placenta, by which the embryo lives upon and breathes through 

 the uterine blood of the mother. From an early period up 

 to birth the placental circulation is the chief, we may almost 

 sa} T the only means by which the embryo breathes and is fed ; 

 but the details of the placental events are changing during the 

 whole of this time. The embryo, all the while increasing in 

 bulk, passes through phase after phase ; the structural features 

 of one day give way to those of the next, its morphological 

 history being as it were a series of dissolving views ; and each 

 new structural phase entails new functional events both in 

 the embryo itself and in the placenta. This is perhaps especially 

 seen in the earlier stages at a time when the placental circula- 

 tion has been established in its main outlines, but in the embryo 

 most of the future organs are still in a shadowy inchoate condi- 

 tion. At this epoch, of the total bulk of blood coursing 

 from the embryo towards the tissues of the mother and back 

 again, the greater part is at any one moment to be found in the 

 placenta and only a small part in the tissues of the embryo itself ; 

 later on the blood is equally divided between the placenta and 

 the embryo ; and still later the embryo has the larger share, and 

 it is the smaller part which is at any one moment flowing 

 through the chorionic villi of the placenta. There can be no 

 doubt that in the earlier phase the influences which the placental 

 structures exert on the foetal blood are in many ways different 

 from those which are exerted later on. We find that during the 

 earlier phases the cellular placental elements are correspondingly 

 prominent, indicating that much labour of the kind for which cells 

 are necessary is being then carried on, whereas in the later stages 

 the placental mechanism approaches though it never quite 

 reaches the more mechanical conditions of a simple membrane 

 separating the foetal and maternal blood. We cannot enter at all 

 fully here into the successive phases ; we must confine ourselves 

 chiefly to the main features of what is going on during the latter 

 months of gestation when the placental circulation is in full swing. 

 § 697. At this time the somewhat rapid strokes of the foetal 

 heart drive the foetal blood through the umbilical arteries to the 

 capillaries of the chorionic villi, from whence it is returned 

 by the umbilical vein. From experiments on lambs and other 

 animals it would appear that the blood pressure in the umbilical 

 artery is moderately high ( 40 to 80 mm. Hg.) and that in the 

 umbilical vein very considerable (15 to 30 mm. Hg.), higher 

 than the venous pressure in the mother in a vein of correspond- 



