DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 181 



willing to deny it. So much the more positively do WENKEBACH 

 and ZIEGLER, on the strength of their investigations on Teleosts, 

 express themselves against the mode of blood-formation given by 

 GENSCH. According to ZIEGLER, the blood-corpuscles are developed 

 in the blood-vessels of the embryonic body itself. The free nuclei 

 of the yolk, the merocytes, on the contrary, it is maintained, do not 

 share in the formation of embryonic tissues, but, in adaptation to 

 the function of resorbing the yolk, undergo peculiar modifications, 

 which " cause the frequently affirmed but never proved production 

 of blood-corpuscles [by them] to appear improbable." 



Under this condition of affairs, I must regard the question of the 

 source of the cell-layer in which, in the region of the opaque area, 

 the formation of blood takes place as not yet ready for final 

 judgment. 



So far as regards the further changes, by means of which the 

 cell-layer under consideration is converted into connective substance 

 and blood, on the whole I subscribe, in this difficult field of in- 

 vestigation, to KOLLIKER'S representation. 



At the end of the first day of incubation, the masses of cells which 

 lie between the inner and the outer germ-layers arrange themselves 

 in cylindrical or irregularly limited cords, which join themselves to- 

 gether into a close-meshed network ; they are the first fundaments 

 both of the vessels and also of their contents, the blood. In the 

 spaces of the net are to be found groups of indifferent cells, which 

 afterwards become embryonic connective tissue, and which are the 

 Substanzinseln (fig. 114) of authors. 



At the beginning of the second day of incubation, the solid funda- 

 ments of the vessels become more distinct, in proportion as they 

 become bounded superficially by a special wall, and acquire 

 an internal cavity. The wall of the vessels is developed out of 

 the most superficial cells of the cords, and is composed during the 

 first days of incubation of a single layer of very much flattened 

 polygonal elements, on account of which the first vessels of the 

 embryo are often designated as endothelial tubes (fig. 114 and 

 fig. 115 gw). 



The cavity of the vessel is probably formed by the penetration of 

 fluid into the originally solid cord from its surroundings, thus forming 

 the plasma of the blood, by which the cells are pressed apart and to 

 the sides. The cells then constitute here and there thickenings of 

 the wall, and project into the fluid-filled cavities as elevations of 

 loosely united spherical elements (fig. 114, Blood-islands). Conse- 



