GENERAL EMBRYOLOGY 1641 



of their cells, eventually disappear. The erUhroblasts are at first 

 colourless, but soon assume a straw-colour (pale yellow), and they 

 constitute the primitive red corpuscles, being the only globules 

 which the blood contains at this stage. Each has a large nucleus 

 at this period. Being nucleated cells, they undergo mitotic division 

 ior some time, and so multiply. 



In the second month of intra-uterine life, however, non-nucleated 

 red corpuscles appear, known as erythrocytes, which rapidly increase 

 in number. Many of the er\-throblasts lose their nuclei, probably 

 by a process of extrusion, 'and then become non-nucleated red 

 corpuscles. It is to be noted that, though in early life the red 

 corpuscles are nucleated, in adult life they are non-nucleated in 

 Mammalia. 



The white corpuscles or leucocytes are formed outside the blood- 

 vessels, and therefore enter the blood-stream by immigration. The 

 sources from which they are derived are as follows: (i) The spleen; 

 (2) the marrow of bone ; (3) the l^Tuphatic glands, IjTnphoid follicles, 

 tonsils, and th>-mus; and (4) the embryonic liver. 



The blood-platelets may represent portions of disintegrated leuco- 

 cytes, cr they may be fragments of the multi-nucleated giant-cells 

 of bone-marrow. 



The organs concerned in the formation of blood-corpuscles are 

 spoken of as hcematopoietic (blood-forming) organs. 



The process of vascularization invades the entire wall of the yolk- 

 sac, and is brought about in a manner similar to that which afiects 

 the vascular area. The splanchnopleure is invaded by cell-cords; 

 the superficial cells of these cords form the endothelial linings of 

 the vessels; and the contained ceU-cord, in each case, becomes 

 hollowed out to form the lumen of the vessel. 



This process of vascularization extends along the vitelline duct, 

 and in this manner the vessels are brought within the body of the 

 embryo. Thereafter they become connected with the primitive 

 heart, which at this stage is undergoing development. \Vhen this 

 connection has been effected — direct as regards the vitelline veins, 

 but indirect as regards the viteUine arteries — ^the vitelline circula- 

 tion is established, by means of which nutritive materials are con- 

 veyed from the yolk-sac to the embryo (chick or rabbit). 



The Vitelline Circulation. 



The bloodvessels concerned in the vitelline circulation are four in 

 number — ^namely, (i) the two vitelline or omphalo-mesenteric veins, 

 and (2) the two vitelline or omphalo-mesenteric arteries — ^there 

 being an intervening system of vessels, of the nature of capillaries, 

 and a blood-channel, called the sinus terminalis. 



The sinus terminalis surrounds the vascular area of the yolk-sac, 

 and the numerous capillar^' vessels proceeding from it give rise by 

 their confluence to the two vitelline veins. These veins, having 

 entered the body of the embryo, pass along the primitive intestinal 



