320 HISTORY OF THE HUMAN BODY 



are most of the others, and that is, because this system is 

 actively functional almost from the beginning of embryonic 

 life, while in other cases the parts lie passive and let themselves 

 gradually assume the -final shape without contributing anything 

 to the functional life of the organism, a condition most favor- 

 able to the suppression of intermediate stages. 



The early vertebrate embryo, during its cleavage stages, ap- 

 pears most frequently as a circular disc of rapidly proliferating 

 cells floating on the surface of a spherical or spheroidal yolk- 

 mass ; and although at first these cells possess sufficient energy 

 within themselves to continue development, there soon comes 

 a time at which they become dependent upon the nutriment 

 stored in the yolk, and it is thus one of the earliest cares of 

 the organism to develop blood-vessels for the purpose of carry- 

 ing yolk granules to the embryonic area. 



These blood-vessels first appear as irregularly branching 

 spaces on the surface of the yolk beyond the limit of the 

 definite embryo; these spaces soon form themselves into a 

 capillary net-work and unite upon each side of the -embryo 

 into a vitelline vein. 



Within the embryo a similar process lays down the first 

 blood-vessels and the entire system appears as in Fig. 89, A. 

 The two vitelline veins unite into a median vessel, the future 

 heart, situated ventrally with reference to the embryo, and 

 immediately back of the future gill region. Further anteriorly 

 the median vessel divides and forms two lateral loops, the 

 first arterial gill-arches, which continue around the pharynx 

 until they come in contact with one another upon the ventral 

 side of the notochord, from which point they run backwards, 

 forming two aorta. 



At a point a little posterior to the entrance into the embryo 

 of the vitelline veins, the aortse pass mainly into the forma- 

 tion of two vitelline arteries, which spread out over the yolk, 

 but the small vessels which continue into the posterior end 

 of the embryo form morphologically their real continuation. 

 During later development the posterior aorta fuse into one 

 and increase greatly in size so that the proportions between 



