DEVELOPMENT OF VASCULAR SYSTEM. 597 



united, as if webbed for swimming ; but this is to be regarded 

 not so much as an approximation to the form of aquatic ani- 



FlG. 231. 



'if 



A human embryo of the fourth week, 3^ lines in length. 1, the chorion ; 3, part 

 of the amnion ; 4, umbilical vesicle with its long pedicle passing into the abdomen ; 

 7, the heart ; 8, the liver ; 9, the visceral arch destined to form the lower jaw, be- 

 neath which are two other visceral arches separated by the branchial clefts ; 10, ru- 

 diment of the upper extremity; 11, that of the lower extremity; 12, the umbilical 

 cord ; 15, the eye ; 16, the ear ; 17, the cerebral hemispheres ; 18, the optic lobes or 

 corpora quadrigemina. 



mals, 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. 588). 

 The earliest form of the heart presents itself as a solid com- 

 pact mass of embryonic cells, similar to those of which the 

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

 provided with a cavity ; but this shortly makes its appearance, 

 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 float- 

 ing 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 commencing pulsations of the heart. These pul- 

 sations take place even before the appearance of a cavity, and 

 immediately 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 



