252 



BREEDING 



no actual communication between the two sets of capillaries, but the blood- 

 stream of the mother and that of the foetus are separated only by the thin 

 walls of the vessels, through which the blood is constantly flowing. The 

 interchange which takes place between the maternal and the foetal blood, for 

 the nutrition of the young animal, necessarily is carried on through the two 

 layers of membrane by osmosis, i.e. that force which regulates the inter- 

 change of fluids through wet membranes. 



Blood-vessels in the embryo commence by formation of a thin mem- 

 brane in the blastoderm, between the serous and mucous layers, at a part 

 which is described as the vascular area. Eed lines appear, and form a net- 



Fig. 540. — Development of the Human Ovum 



1. Early stage: «, interior and exterior folds of the serous layer joining the amnion; 

 h, embryo; c, incipient allantois; rf, chorion; e, vitelline mass surrounded by blastodermic 

 vesicle. 2. Second month: «, amnion, outer layer, coalescing with chorion; h, embryo; 

 c, umbilical vesicle; d, amnion, inner layer; e, smooth portion of chorion; /, villous 

 portion of chorion ; g, elongated villi collecting into placenta. 



work of vessels filled with blood, a rudimentary heart is formed in the 

 vascular area, and to that organ the branching vessels proceed, and the 

 outline of the circulatory system is complete ; the details being filled in 

 by further developments in correspondence with the continuous advance of 

 the embryonic structures. 



In the next illustration (fig. 541) the condition of the embryo and its 

 membrane at the age of seven weeks is shown. 



Changes which occur in the Uterus in Gestation.— Further 



consideration of embryonic growth and development may be deferred for 

 a space, in order to explain the adaptive alterations which have up to this 

 time taken place in the uterus. 



At an early period in utero-gestation the openings of the glands of the 

 mucous membrane lining the uterus increase in size and become more 

 numerous. Meanwhile the membrane itself receives additions which render 

 it softer, thicker, and more vascular than the normal membrane; in fact, 

 the added materials constitute a new membrane under the name of the 



