70 



THE SECOND DAY. 



[CHAP. 



situate in the envelopes of the nodal groups, as well as those lying on the exterior 

 of the connecting processes, appropriate a quantity of the granular protoplasm 

 surrounding each, and thus become converted into spindle-shaped cells. Each 

 nodal group and each connecting process thus gets a distinct wall of nucleated 

 cells. By the continued widening of the connecting processes and solution of their 

 central portions, accompanied by a corresponding increase in the enveloping 

 nucleated cells, the original protoplasmic network is converted into a system 

 of communicating tubes, the canals of which contain blood-corpuscles and 

 plasma, and the walls of which are formed of spindle-shaped nucleated cells. 



FIG. 19. 



I.e.- 



SURFACE VIEW FROM BELOW OF A SMALL PORTION OF THE POSTERIOR END OF 

 THE PELLUCID AREA OF A 36 HOURS' CHICK. To illustrate the formation of 

 the blood- capillaries and blood-corpuscles, magnified 400 diameters. 



6. c. Blood- corpuscles at a nodal point, already beginning to acquire a red 

 colour. They are enclosed in masses of protoplasm in the outermost 

 layer of which are found nuclei, a, some of which contain two nucleoli. These 

 nuclei subsequently become the nuclei of the cells forming the walls of 

 the vessels. The nodal groups are united by protoplasmic processes (p.pr), 

 also containing nuclei with large nucleoli (ri). These nuclei increase in 

 number by division, and become converted in part into the nuclei of the 

 cells forming the walls of the vessels, and in part into blood-corpuscles. 



The blood-corpuscles pass freely from the nodal points into the hollow pro- 

 cesses, and thus the network of protoplasm becomes a network of blood-vessels ; 

 the corpuscles and the nuclei of the walls of which have been by separate 

 paths of development derived from the nuclei of the original protoplasm. 



The formation of the corpuscles does not proceed equally rapidly or to the 



