CHAPTER X. 



THE DEVELOPMENT OF THE VASCULAR SYSTEM. 

 THE BLOOD VESSELS AND BLOOD. 



Inasmuch as the blood vessels form such an extensive and extremely com- 

 plicated system, it would be obviously beyond the scope of this book to con- 

 sider the details of the development of all parts of this system. A sorn 

 detailed discussion of the heart and of the larger vascular trunks will bi 

 but in the case of the smaller vessels very brief statements must suffice. 



The Heart. 



Despite the fact that in the adult the heart is so intimately associated with 

 the blood vessels and blood, it begins to develop quite independently, and is 

 said to begin to beat before the vessels containing the blood are connected with 

 it. It is one of the first organs to appear, and at a very early stage assum- 

 function which it maintains throughout the life of the organism. The primitive 

 heart is a very simple structure^ consisting merely of a tube who 

 capable of contractile activity. One end of this tube is attached to the a \ 

 m and the other to the venous system. This simple form is elab< 

 more and more during development until it reaches the complicated str 

 characteristic of the adult. 



The heart has a peculiar origin in that it two separate par 



anlagen which unite secondarily. In the chick, for example, it appe. 

 the first day of incubation, at a time when the germ-layers are still )i:it. The 

 r. in the cephalic region becomes dilated to form the so-called 



fty (parietal cavity), and at the same time a spa, 

 ide, not far from the medial line, in { vrmal layer of the 



pleure (Fig. 194). The- .ed with a gelatinous si 



whid ' ited cells. These cells then take on the appearance 



dothelium and line the cavities, and the mesothclium in tnis vicinity is ch. 

 into a distinct, thickened layer of cells. Now by at; 



re the nm'tics or 



The bending ccatir* - ' ; . of each 



, that of the 0| vvity -the fc>; 



orm ventral to the err vay and allows the medial walls 



endothelial tube- ro t p >,iUict. These wail.- then bi 



and ti i uniti -:H! ventral line to f i-^le tube fFiii. 



o \ O 



