DEVELOPMENT OF VASCULAR SYSTEM. 765 



much as an approximation to the form of aquatic animals, 

 as the primitive form of the hand, the individual parts of 

 which subsequently become more completely isolated. 



Development of the Vascular System. 



The first development of the vascular system and heart in 

 the germinal membrane has been already alluded to (p. 751). 

 The earliest form of the heart presents itself as a solid 

 compact mass of embryonic cells, similar to those of which 

 the other organs of the body are constituted. It is at first 

 unprovided with a cavity ; but this shortly makes its ap- 

 pearance, resulting apparently from the separation from 

 each other of the cells of the central portion. A liquid 

 is now formed in the still closed cavity, and the central 

 cells may be seen floating within it. These contents of 

 the cavity are soon observed to be propelled to and fro 

 with a tolerable degree of regularity, owing to the com- 

 mencing pulsations of the heart. These pulsations take 

 place even before the appearance of a cavity, and imme- 

 diately after the first 'laying down' of the cells from 

 which the heart is formed. At first they seldom exceed 

 from fifteen to eighteen in the minute. The fluid within 

 the cavity of the heart shortly assumes the characters of 

 blood. At the same time the cavity itself forms a com- 

 munication with the great vessels in contact with it, and 

 the cells of which its wall are composed are transformed 

 into fibrous and muscular tissues, and into epithelium. 



Blood-vessels appear to be developed in two ways, 

 according to the size of the vessels. In the formation of 

 large blood-vessels, masses of embryonic cells similar to 

 those from which the heart and other structures of the 

 embryo are developed, arrange themselves in the position, 

 form and thickness of the developing vessel. Shortly after- 

 wards the cells in the interior of a column of this kind 

 seem to be developed into blood- corpuscles, while the 



