CHAPTER XL 



The Vascular System and Vascular Glands. 



The present chapter deals with the early development of 

 the heart, the development of the general circulatory system, 

 especially the venous part of it, and the circulation of the 

 yolk-sack. It also contains an account of two bodies which I 

 shall call the suprarenal and interrenal bodies, which are 

 generally described as vascular glands. 



The heart 



The first trace of the heart becomes apparent during stage 

 G, as a cavity between the splanchnic mesoblast and the wall 

 of the gut immediately behind the region of the visceral clefts 

 (PL X. fig. 4, ht). 



The body-cavity in the region of the heart is at first double, 

 owing to the two divisions of it not having coalesced ; but even 

 in the earliest condition of the heart the layers of splanchnic 

 mesoblast of the two sides have united so as to form a 

 complete wall below. The cavity of the heart is circumscribed 

 by a more or less complete epithelioid (endothelial) layer of 

 flattened cells, connected with the splanchnic wall of the heart 

 by protoplasmic processes. The origin of this lining layer I 

 could not certainly determine, but its connection with the 

 splanchnic mesoblast suggests that it is probably a derivative 

 of this\ In front the cavity of the heart is bounded by the 

 approximation of the splanchnic mesoblast to the wall of the 

 throat, and behind by the stalk connecting the alimentary canal 

 with the yolk-sack. 



1 From observations on the development of the heart iu the Fowl, I have 

 been able to satisfy myself that the epithelioid lining of the heart is derived 

 from the splanchnic mesoblast. When the cavity of the heart is being formed 

 by the separation of the splanchnic mesoblast from the hypoblast, a layer of 

 the former remains close to the hypoblast, but connected with the main mass of 

 the splanchnic mesoblast by protoplasmic processes. A second layer next be- 

 comes split from the splanchnic mesoblast, connected with the first layer by the 

 above-mentioned protoplasmic processes. These two layers form the epithelioid 

 lining of the heart; between them is the cavity of the heart, which soon loses 

 the protoplasmic trabcculae which at first traverse it. 



