182 



EMBRYOLOGY. 



fe " Blood-island 



Wall of blood- 

 vessel 



quently the vessels which are just becoming permeable are very 

 irregular, since narrow places and wider ones, often provided 



with evagina- 

 tions, alternate 

 (fig. 114) with 

 one another, 

 an d since 

 the vessels 

 are sometimes 

 wholly excava- 

 ted, fluid-filled, 

 endothelial 

 tubes, and 

 sometimes re- 

 main more or 

 less impassable, 

 owing to the 

 variously 

 formed cell ag- 

 gregates which 

 project from 

 the wall. 



The aggrega- 

 tions of cells 

 themselves are 

 simply the 

 centres where 

 the formed com- 

 ponents of the 

 blood are pro- 

 duced. The 

 small spherical 

 nucleated cells, 

 which still en- 

 close dark yolk- 

 granules, be- 

 come at first 

 homoge neons 



Blood-island 



Blood-vessel 



Wall of blood- 

 vessel 



Substanzinseln 

 Blood-vessel 



Fig. 114. A portion of the vascular area of the germ-disc of an embryo 

 Chick, in which 12 primitive segments are developed, after DISSE. 



One sees the more darkly shaded blood-coimes, in which lie the 

 " blood- islands," the centres whence the blood-corpuscles arise- 

 The clear spaces in the vascular network, the walls of which are 

 formed of flat endothelial cells, are the " substance-islands " 

 (Substanzinselu). 



by the dissolution of the latter, and then, owing to the formation 

 of the coloring matter of the blood in them, they take on a slightly 

 yellowish color, which gradually becomes more intense. 



