DEVELOPMENT OF BLOOD-VESSELS AND CORPUSCLES. 687 



embryo and, after the appearance of the large converging trunks, the vitelline veins 

 and arteries, joins the intra-embryonic trunks that coincidently have been formed. 



Although the generally accepted current views relating to the independent origin 

 of the primary blood-vessels within the vascular area have not escaped challenge, it 

 may be regarded as established that the development of subsequent blood-vessels 

 proceeds from the cells constituting the walls of pre-existing channels. The walls 

 of the growing capillaries consist of delicate endothelial plates from which pointed 

 sprouts grow into the surrounding tissue (Fig. 651). These outgrowths, direct pro- 

 longations of the cytoplasm of the endothelial cells, are at first solid, but later become 

 hollowed out by the gradual extension of the lumen of the capillary. Vascular loops 

 are often formed by the meeting and fusion of the outgrowths proceeding in opposite 

 directions, the communication being established by the final disappearance of the sep- 

 tum in consequence of the extension of the lumen of the parent vessels. At first rep- 

 resented by only a single layer of 



endothelial cells, the walls of the Fig. 650. 



larger blood-vessels become rein- 

 forced by the additional layers 

 derived from the surrounding 

 mesoblast. 



Development of the 

 Erythrocytes. — The first, and 

 for a time the only, blood-cells 

 present within the embryo are 

 the primary nucleated erythro- 

 cytes derived probably directly 

 from the mesoblastic elements 

 within the angioblastic areas in 

 which the earliest vessels appear. 

 These cells, the primary erythro- 

 blastSy separated by the colorless 

 plasma which appears between 

 them, undergo mitotic division, 

 producing nucleated elements 

 that, in turn, give rise to the 

 primary erythrocytes. These 

 are spherical, nucleated, and 

 larger (about .012 mm. in diam- 

 eter) than the adult red cells. 

 At first their cytoplasm is color- 

 less and slightly granular, but 

 soon becomes homogeneous and 

 tinged with hemoglobin. 



After the earlier foetal 

 months, during which prolifera- 

 tion of the blood-cells occurs 

 in all parts of the circulation, 

 the corpuscles engaged in division withdraw to localities in which the blood-current 

 is sluggish and, therefore, favorable for mitosis. Such localities are particularly the 

 liver, spleen, and bone-marrow, the large capillaries and tissues of which afford tem- 

 porary resting places during proliferation. From the primary blood-cells arise viegal- 

 oblasts and norm.oblasts , from which latter the definite erythrocytes are derived. 

 These changes begin during the second foetal month, more and more nonnucleated 

 discoidal red cells appearing as gestation advances, so that at birth almost all the 

 nucleated erythrocytes have disappeared from the circulation. 



Since the red cells possess only a limited vitality, their constantly occurring 

 death requires the production of new corpuscles. Preceding the development of the 

 spleen and bone-marrow, the liver is the principal centre of blood-formation. Later 

 the splenic and marrow tissues share this function, while after birth the red bone- 

 marrow is the chief seat in which the continual additions of new erythrocytes necessary 



■WjlSti^^^j^ 



Surface view of vascular area of chick embryo with twelve 

 somites (29 hours) ; net-work of developing blood-vessels, most 

 distinct in periphery of area, is connected with vitelline veins 

 from embryo by faint channels ; cephalic segment of neural 

 tube shows brain-vesicles and eye-buds ; caudal segment still 

 widely open. X 16. 



