196 TEXT-BOOK OF EMBRYOLOGY. 



slight supremacy over its fellows, would gradually cut its way deeper into 

 the soil and pursue a straighter course, with the result that the other rivulets 

 would flow into it as the main channel. 



The concept that the main vascular trunks are preceded by a capillary 

 plexus, out of which they develop in response to certain mechanical stimuli, 

 offers a simple explanation of the numerous variations found in the vascular 

 system. In the incipient stages of the larger vessels but slight influences, 

 due to variations in the development of surrounding structures, would be 

 sufficient to deflect their courses and cause them to occupy positions which 

 do not accord with the normal. So far as the thickened walls of the larger 

 vascular channels are concerned, they may be regarded as structural adapta- 

 tions to the functions they perform. For example, the large amount of 

 elastic tissue in the wall of the aorta and other large arteries tends to main- 

 tain a uniform diameter in these vessels against the force exerted by the 

 blood expelled from the heart at each contraction. 



The Heart. The heart has a peculiar origin in that it arises as two sep- 

 arate 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 splanchnopleure (Fig. 165). 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 endothelium and line the cavities, and the meso- 

 thelium 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 mid ventral line (Fig. 165). 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. 165), which extends 

 longitudinally for some distance in the cervical region of the embryo. The 

 mesothelial layers of opposite sides meet dorsal and ventral to the endo- 

 thelial tube, forming the dorsal and ventral mesocardium (Fig. 165). In 

 the meantime the cephalic end of the tube has united with the arterial system, 

 and the caudal end with the venous system ; and in a short time the dorsal 

 and ventral mesocardia disappear and leave the heart suspended by its 

 two ends in the primitive pericardial cavity. The conditions at this point 

 may be summarized thus: The heart is a double-walled tube the inner wall 

 composed of endothelium and destined to become the endocardium, the 



