KllVE SUBSTANCE AND BL<>oj> 181 



to deny it. So much tin- lively do \\KNKEBACH 



ami Zn:<;LER, on the stren<;tli .1' ihn'r in\->t i^at ions on Teleosts, 

 -\piv>s t In -ms -Ivrs against the mode of blood-formation given by 

 GENSCH. According to ZIBGLIB, the blood-oorpoflolei are developed 

 in the l>lood-ves>els of the embryonic bodv it>-lf. The free nuclei 

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

 share in the formation of embryonic i.ut, in adaptation i< 



the function of resorbing the yolk, undergo peculiar modifican 

 which " cause the frequently affirmed but never proved production 

 of Mood 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 ditlicult 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 them>- 

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

 ri nto a close-meshed network; they are the first fundaments 

 both of the vessels and also of then- contents, the blood. In the 

 spaco of the net are to be found groups of indifferent cells, wl.ich 

 afterwards become embryonic connective tissue, and which are the 

 iisehi (fig. 114) of autli' 



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 lirst vessels of the 

 eml.rvo are often designated as endothelial tubes (fig. 114 

 fig. 115 gw). 



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

 lluid into the originally solid cord from its Mirrouudings, 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 thickeniii^ 

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

 loosely united spherical elements (liir. Ill, Blood-islands). Conse- 



