638 REPRODUCTION. 



vascular plexus in which the blood performs a continuous circulatory 

 movement. The heart is at this time a bent tube giving origin to two 

 main arteries, the aortae, which, after curving backward, run on each 

 side, for nearly the whole length of the body of the embryo, beneath 

 the protovertebraB, in the inferior lamina of the mesoderm. (Figs. 185 

 and 186, ao). Near the posterior extremity of the pro vertebral chain 

 they supply two large branches, the vitelline arteries, which pass out, 

 one on each side, to ramify in the area vasculosa. When first formed 

 these arteries are of wide calibre, and their branches communicate with 

 each other by frequent inosculations, both in the immediate neighbor- 

 hood of the embryo and in the area vasculosa,, The vena terminalis, 

 after making the circuit of the area vasculosa, curves backward in front 

 of the embryo on each side, near the median line, forming the anterior 

 vitelline veins, right and left. Those veins are not only supplied with 

 blood from the vascular plexus, but also receive inosculating branches 

 from the adjacent portions 01 the vena terminalis. They continue back- 

 ward to a point just behind the situation of the heart, where they 

 unite with two veins coming from the sides, the lateral vitelline veins, 

 by which the blood is finally returned from the area vasculosa to the 

 venous extremity of the heart. 



The circulation of the area vasculosa transfers to the embryo the 

 nutritious fluids of the vitellus. The blood distributed over the surface 

 of the yolk absorbs its organic materials and returns with them by the 

 vitelline veins ; and by a continuation of this movement the nutritive 

 substances stored up in the yolk sac are utilized for the growth of the 

 embryonic tissues. After the establishment of this circulation, accord- 

 ingly, the development of the embryo goes on with increased rapidity. 

 The subsequent changes vary in different species and classes according 

 to the final disposition and relative importance of various parts ; but in 

 all vertebrate animals the origin and early formation of the organs 

 follow a similar course to that in the fowl's egg during incubation. 



