182 



EMBRYOLOGY. 



I'.lo, id-isl.-md 



Wall of 1.1, -,. 



vessel 



quently the vessels which are just becoming permeable are very 

 irregular, since narrow places and wider ones, often provided 



with 



v ; 



\v 

 Si 



tions, alternate 

 (fig. 114) with 

 one another, 

 and since 

 the vessels 

 are sometimes 

 wholly excava- 

 ted, fluid-filled, 

 en d o t heli al 

 tubes, a n d 

 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 n e o u s 



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-courses, in which lie the 

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

 The clear spaces in the vascular netwoi'k, 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. 



