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 somewhat 

 detailed discussion of the heart and of the larger vascular trunks will be given 

 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 assumes the 

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

 heart is a very simple structure, consisting merely of a tube whose wall is 

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

 system and the other to the venous system. This simple form is elaborated 

 more and more during development until it reaches the complicated structure 

 characteristic of the adult. 



The heart has a peculiar origin in that it arises as two separate parts or 

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

 the first day of incubation, at a time when the germ layers are still flat. The 

 ccelom in the cephalic region becomes dilated to form the so-called primitive 

 pericardial cavity (parietal cavity), and at the same time a space appears on 

 each side, not far from the medial line, in the mesodermal layer of the splanchno- 

 pleure (Fig. 194). These spaces at first are filled with a gelatinous substance in 

 which lie a few isolated cells. These cells then take on the appearance of en- 

 dothelium and line the cavities, and the mesothelium in this vicinity is changed 

 into a distinct, thickened layer of cells. Now by a bending ventrally of the 

 splanchnopleure the cavities or vessels are carried toward the midventral line 

 (Fig. 194). The bending continues until the entoderm of each side meets and 

 fuses with that of the opposite side, thus closing in a flat cavity the fore-gut. 

 The entoderm ventral to the cavity breaks away and allows the medial walls 

 of the two endothelial tubes to come in contact. These walls then break away 

 and the tubes are united in the midventral line to form a single tube (Fig. 194), 



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