DEVELOPMENT. 795 



swimming, crawling, walking, or flying. In the human foetus the fin- 

 gers 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 primitive 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). 



The Vascular System. At an early stage in the development of 

 the embryo-chick, the so-called area vasculosa begins to make its appear- 

 ance. 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 nuclei acquire red color ; these form the 

 red blood-corpuscles. 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 com- 

 mences. 



According to Klein, the first blood-vessels in the chick are developed 

 from embryonic cells of the mesoblast, which swell up and become vacuo- 

 lated, while their nuclei undergo segmentation. These cells send out proto- 

 plasmic 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 early period the heart makes its appearance 

 as a solid mass of cells of the splanchnopleure in the manner before indi- 

 cated. 



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 

 that the heart begins to be developed. The heart when first formed is 

 made up of two not quite complete tubes which coalesce to form one, and 

 so when the cavity is hollowed out in the mass of cells, the central cells 

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

 rhythmic 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 



