CHAPTER XI. 

 THE VASCULAR SYSTEM AND VASCULAR GLANDS. 



THE present chapter deals with the early development of the 

 heart, the development of the general circulatory system, es- 

 pecially 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 11, fig. 4,^.). 



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 com- 

 plete 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 pro- 

 toplasmic 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 1 . In 



1 From observations on the development of the heart in 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 



