94 THE SECOND DAY. [CHAP. 



plasmic network is converted into a system of communicating 

 tubes, the canals of whicli contain blood-corpuscles and plasma, 

 and the walls of which are formed of flattened nucleated cells. 



The blood-corpuscles pass freely from the nodal points into 

 the hollow processes, and thus the network of protoplasm be- 

 comes a network of blood-vessels, the nuclei of the corpuscles and 

 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 same extent in all parts of the blastoderm. By 

 far the greater part are formed in the vascular area, but some 

 arise in the pellucid area, especially in the hinder part. In the 

 front of the pellucid area the processes are longer and the network 

 accordingly more open ; the corpuscles also are both later in 

 appearing and less numerous when formed. 



Assuming the truth of the above account, it is evident that 

 the blood-vessels of the yolk-sack of the chick do not arise as 

 spaces or channels between adjacent cells of the mesoblast, but 

 are hollowed out in the communicating protoplasmic substance 

 of the cells themselves. The larger vessels of the trunk are 

 however probably formed as spaces between the cells, much as is 

 the case with the heart. 



WolflSan duct. About this period there may be 

 seen in transverse sections, taken through the embryo 

 in the region of the seventh to the eleventh somite a 

 small group of cells (Fig. 34, W. d) projecting on either 

 side from the mass of uncleft mesoblast on the outside 

 of the mesoblastic somites, into the somewhat triangular 

 space bounded by the epiblast above, the upper and 

 outer angle of the mesoblastic somite on the inside, 

 and the somatic mesoblast on the outside. 



This group of cells is the section of a longitudinal 

 ridge, the rudiment of the Wolffian duct or primitive 

 duct of the excretory system ; while the mass of cells 



