276 HAND-BOOK OF PHYSIOLOGY. 



is nearly the same in all Vertebrata, whether it be destined for swim- 

 ming, crawling, walking, or flying. In the human foetus the fingers are 

 at first united, as if webbed for swimming; but this is to be regarded not 

 so much as an approximation to the form of aquatic animals, as the prim- 

 itive form of the hand, the individual parts of which subsequently become 

 more completely isolated. 



. The fore-limb always appears before the hind-limb and for some time 

 continues in a more advanced state of development. In both limbs alike, 

 the distal segment (hand or foot) is separated by a slight notch from the 

 proximal part of the limb, and this part is subsequently divided again by 

 a second notch (knee or elbow- joint). 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



At an early stage in the development of the embryo-chick, the so- 

 called "area vasculosa" begins to make its appearance. A number of 

 branched cells in the mesoblast send out processes which unite so as to 

 form a network of protoplasm with nuclei at the nodal points. A large 

 number of the nuclei acquire a red color; these form the red blood-cells. 

 The protoplasmic processes become hollowed out in the centre so as to 

 form a closed system of branching canals, in the walls of which the rest 

 of the nuclei remain imbedded. In the blood-vessels thus formed, the 

 circulation of the embryonic blood commences. 



According to Klein's researches, the first blood-vessels in the chick 

 are developed from embryonic cells of the mesoblast, which swell up and 

 become vacuolated, while their nuclei undergo segmentation. These 

 cells send out protoplasmic processes, which unite with corresponding 

 ones from other cells, and become hollowed, give rise to the capillary wall 

 composed of endothelial cells; the blood-corpuscles being budded off from 

 the endothelial wall by a process of gemmation. 



Heart. About the same time the heart makes its appearance as a 

 solid mass of cells of the splanchnopleure. 



At this period the anterior part of the alimentary tube ends blindly 

 beneath the notochord. It is beneath the posterior end of this "fore-gut" 

 (as it may be termed) that the heart begins to be developed. A cavity is 

 hollowed out longitudinally in the mass of cells; the central cells float 

 freely in the fluid, which soon begins to circulate by means of the rhyth- 

 mic pulsations of the embryonic heart. 



These pulsations 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, and long before muscular fibres or ganglia have been 

 formed in the cardiac walls. 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 



