782 COMPARATIVE ANATOMY 



tinct layer of cells. Thirdly, in the middle and lateral parts of the area 

 pellucida, cells are budded off from the upper side of the entoblast to be- 

 come mesoblast, at about the very time the primitive streak is forming. 



Now, all of these mesoblastic cells together unite to form a con- 

 tinuous layer. This layer continues expanding until it passes beyond 

 the boundaries of the area pellucida and forms a middle layer in the 

 inner zone of the area opaca. This zone is the vascular area (Fig. 264). 

 It is in this area that the blood vessels begin to form. This occurs in the 

 chick on the very first day. A network appears in the entire vascular 

 area which surrounds the embryo. Here irregular reddish blotches are 

 formed which are called blood islands, and it is from these blood islands 

 that the red corpuscles are formed. This network develops into a sys- 

 tem of cords, at first solid, but soon a lumen is acquired and, as the ves- 

 sels unite, there is a continuous but indefinite blood vessel formed. The 

 very first vessel which becomes definitely shaped so that it can be rec- 

 ognized as a part of the vascular system is formed around the entire 

 vascular area as a sort of boundary and is called the sinus terminalis. 

 (Fig. 284, C.) 



The blood islands appear in cross section as little local thickenings 

 on the dorsal walls of the blood vessels. These bud off into the cavities 

 of the vessels and form the first blood corpuscles, and it is supposed 

 that from these all the colored corpuscles of the blood are descended. 



The network of vessels continues to grow, some of the vessels later 

 becoming arteries, some veins, and still others remain small as capil- 

 laries. These unite and extend toward the embryo, while, within the 

 embryo proper, there has been a growth of the vascular system also, 

 which has extended outward toward this vascular area. All these ves- 

 sels unite to form the entire vascular system. 



Larger vessels of the vascular area unite with the posterior end of 

 the heart which by this time has already commenced to beat. The 

 other vessels unite with the anterior or cephalad end of the heart and 

 these become the arterial system, so that by the end of the second day, 

 in the chick, a complete vascular system has already been formed with a 

 beating heart. 



At first the heart consists of only two longitudinal vessels which 

 are connected at the cephalic end. These spread out caudad like an in- 

 verted "V" (Fig. 283). The arms of this "V" shaped portion soon fuse 

 together and look like an inverted "Y." The cavities of these two fusing 

 tubes remain apart for some time and then form one cavity. That is, the 

 endothelial lining remains separate as two distinct cavities after the mus- 

 cular walls have united. 



On the dorsal surface the muscular walls are incomplete also for a 

 short time, but after complete fusion the walls also are completed. It 

 is the stem of this "Y" which forms the heart. The two diverging arms 

 of the "Y" unite, or rather have united some time before this, so that 



