The laws and mechanics of circulation, with the principle involved in animal movement . luid in these two points or poles of this circulation. This,together with the action in the membranes themselves, andthe heart and vessels, is sufficient for carrying on circulationin the initial stages of embryonic evolution ; but with the in-crease of growth comes increasing difficulty for effecting it:hence the pumping action which is set up in the placenta andwomb, together with the accumulation of amniotic fluid fortransmitting these actions upon the embryo, as describedabove. Thus, everything is in co
The laws and mechanics of circulation, with the principle involved in animal movement . luid in these two points or poles of this circulation. This,together with the action in the membranes themselves, andthe heart and vessels, is sufficient for carrying on circulationin the initial stages of embryonic evolution ; but with the in-crease of growth comes increasing difficulty for effecting it:hence the pumping action which is set up in the placenta andwomb, together with the accumulation of amniotic fluid fortransmitting these actions upon the embryo, as describedabove. Thus, everything is in correspondence—the liquoramnii, the increasing growth of the placenta and the numberof muscles and nerves in the walls of the womb—and so con-tinues till the close of pregnancy. In other words, it all formsa connected whole in the mechanics of the embryonic circu-lation. The absence of a shell wall permits expansion in the PHYSIOLOGY OF THE CIRCULATION. 347 chorion pari passu with the growth of the embryo, while thewomb expands in concert with this action in the chorion Pig. 143.—Diagram of Young Embryo (Chick) and its Vessels, showing circulation ofumbilical vesicles, and also that of allantois, beginning to be formed.—Dalton.
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