THE PEIMITIVE VASCULAE SYSTEM. 63 



The maternal blood-vessels pass from the muscular wall of the uterus into the sub- 

 mucous tissue, and thence into the placenta, where they traverse the maternal portion 

 and the basal plate of the deciclua and open into the intervillous spaces. The 

 arteries usually open on or near the septa and the veins in the intermediate areas. 



In addition, however, to the constituent parts already described, the chorionic 

 part of the placenta contains some strands of maternal tissue, and in the maternal 

 part there are portions of trophoblast. 



The parts of the decidua found in the chorionic part of the placenta are a series 

 of fibrous strands, the remains of parts of the stratum compactum which were not 

 destroyed by the trophoblastic invasion. They are continuous externally with 

 fibrous strands of the maternal part of the placenta, and serve to separate the 

 placenta into a series of lobes, from 15 to 20 in number. 



The portions of trophoblast met with in the maternal part of the placenta are 

 variable pieces of plasrnodium which appear to have wandered from the general 

 mass. They may be found in any of the strata of the maternal part, and even 

 in the submucous tissue. 



At the end of pregnancy, when intra-uterine life terminates, the fused amnion 

 chorion and decidua capsularis are ruptured, in the region of the internal orifice of 

 the uterus, and the amniotic fluid is expelled through the vagina. Next the foetus 

 is extruded, and as soon as it is born it becomes a child. After the child is born 

 it remains attached to the placenta by the umbilical cord (Fig. 80), which is usually 

 ligatured in two places and then divided, between the ligatures, by a medical man 

 or an attendant. Afterwards the placenta is expelled from the uterus. 



Detachment of the placenta is probably caused by contraction of the muscular 

 substance of the uterus, and it takes place by rupture of the strands of the spongy 

 layer of the decidua (Fig. 80). As the detached placenta is expelled the decidua 

 vera is torn through along the line of the spongy layer, and the fused amnion and 

 chorion Iseve and the inner part of the decidua vera, which are attached to the 

 margin of the placenta and which constitute the membranes, are expelled with it. 



At birth the placenta weighs about 500 grm., it has a diameter of about 

 16 to 20 cm., and is about 3 cm. thick. Its inner surface is covered with the amnion 

 which fused with the chorion towards the end of the second month of pregnancy. 

 Its outer surface is rough, it is formed by the remains of the spongy layer of the 

 decidua, and is divided into a number of areas by a series of fissures which 

 correspond in position with the septa by which the organ is divided into lobes. 



THE PKIMITIVE VASCULAE SYSTEM AND THE 

 FCETAL CIECULATION. 



As the zygote travels along the uterine tube, from the ovarian towards the 

 uterine end, it exists either upon the yolk granules derived from the ovum or 

 upon substances absorbed from the fluids by which it is surrounded. After it 

 enters the uterus it must depend, for a time, upon the same sources of nutriment, 

 but as it penetrates the decidua it is probable that the cells of the trophoblast 

 actually devour the cells of the decidua which they invade. This, source of food 

 is only sufficient for a short period, whilst the zygote remains relatively small, 

 and substances absorbed by its surface cells can be transmitted easily to all parts. 



Whilst the period exists, however, not only are the decidual tissues utilised as 

 a food-supply, but fluids are absorbed from them and transmitted into the interior 

 of the zygote to fill the expanding cavities of the amnion and the coelom. 



In all probability the fluids passed into the zygote contain nutritive materials 

 which suffice for the requirements of the embryonic and non-embryonic parts of 

 the zygote so long as both consist of comparatively thin layers of cells, but when 

 the embryonic area increases in thickness, and begins to be moulded into the 

 embryo, its association with adjacent fluids becomes less intimate, and as the 

 development of its various parts progresses, a supply of food and oxygen is required 

 which is greater than can be provided by osmosis from the adjacent fluid media. 

 Thus an imperative necessity arises for a method of food-supply adequate to the in- 

 creasing requirements upon which the continued development and growth depend. 



