PRIMITIVE VASCULAR SYSTEM AND FCTAL CIRCULATION. 57 
When the period of intrauterine life is completed the muscular walls of the 
uterus contract and the lower orifice of the uterine cavity is dilated, the fused 
chorion and amnion, which close the upper part of the orifice, rupture and the 
amniotic fluid escapes, the foetus is then expelled, but it remains attached to the 
placenta by the umbilical cord. The cord is divided artificially, and after a short 
period the placenta and membranes are expelled. The membranes attached to the 
placenta consist of the fused amnion, chorion, decidua capsularis, and also the 
decidua vera internal to the altered spongy layer; therefore both the placenta and 
the membranes consist of maternal and feetal tissues. Before the placenta and 
membranes are expelled the uterine decidua is separated into two parts by a 
cleavage which takes place in the modified stratum spongiosum. The inner portion 
which includes the placenta and membranes is cast off. The outer portion remains 
in the uterus; it consists almost entirely of the deep unchanged layer of the 
decidua, and from it the uterine mucous membrane is reconstructed. 
THE PRIMITIVE VASCULAR SYSTEM AND THE FQTAL 
CIRCULATION. 
It has already been said that the ovum during its passage down the Fallopian 
tube lives either on its own yolk particles or upon substances absorbed from the 
fluids by which it is surrounded. For a time after it enters the uterus its nutrition 
must be provided for in a similar manner, but as soon as the chorionic villi are 
formed it is probable that the ectodermal cells, of which in the earliest stages they 
entirely consist, and which cover their surfaces in the later stages, actually eat up 
the decidual tissues which they invade and use them for food. This source of 
nutrition, however, is only sufficient for the short period during which the ovum 
remains relatively small and substances absorbed through the surface cells can be 
readily transmitted to all its parts. 
In addition to the solid decidual tissues devoured by the ectodermal cells it is 
evident that fluids from the mother are also absorbed, for the yolk-sac and coelom 
enlarge and are filled with fluid. The only sources from which these can have been 
derived are the uterine glands or the blood and lymph vessels of the decidua. 
In all probability the fluids absorbed into the ovum contain nutritive material, 
and so long as the embryo is constituted by the thin layers of the early blastoderm 
sufficient food material can easily be absorbed. When, however, the various parts 
of the embryo increase in thickness and become moulded into the form of or ans 
they are no longer in such intimate relation with the surrounding nutritive fluids, 
whilst, further, as their development progresses they require a greater amount of 
food and oxygen than they can obtain from these fluids. There is, therefore, an 
imperative necessity for a further supply of nutritive material by which their 
requirements may be satisfied, or development must cease and death ensue. 
To meet this necessity the vascular system is formed. It is essentially an 
Irrigation system consisting of a propulsive organ, the heart, and of tubular 
vessels, the blood-vessels, all of which contain blood. The heart propels the blood 
through the blood-vessels to all parts of the embryo, but the blood which is at first 
formed from the mesoderm of the ovum must, at least so far as its fluid part is 
concerned, be supplemented largely from maternal sources. It is necessary, there- 
fore, that the foetal blood-vessels be brought into close relation with the maternal 
plood-vessels at an early period. It is for this purpose, amongst others, that the 
large blood-sinuses are formed in the maternal portion of the placenta, and that 
they are surrounded and invaded by the foetal vill, carrying in their interior 
branches of the foetal blood-vessels, and as previously ahiewnt the foetal blood- 
vessels in the placenta are only separated from the maternal blood in the sinuses 
by their own thin mesodermal walls, and by one or two layers of ectodermal cells. 
When the placenta is fully formed fluids can readily pass from the maternal to 
the foetal vessels, and there can be no doubt that both food and oxygen pass from 
the maternal blood to the fcetal blood through and by the agency of the intervening 
cells, whilst at the same time the waste products which are formed in the embryo 
pass outwards to the maternal blood. 
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