DEVELOPMENT. 



685 



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 splanchno-pleure in the manner before in- 

 dicated. 



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

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

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



FIG. 470. Capillary blood-vessels of the tail of a young larval frog, a, capillaries permeable to 

 blood; 6, fat granules attached to the walls of the vessels, and concealing the nuclei; c, hollow pro- 

 longation of a capillary, endmg in a point; d, a branching cell with nucleus and fat- granules; it 

 communicates by three branches with prolongation of capillaries already formed ; e, e, blood cor- 

 puscles still containing granules of fat. x 350 times. (Kolliker.) 



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 



