Royal Microscopical Society. 61 



mentioned, and regarding the primary formation of the coloured 

 blood corpuscles in the interior of the embryonic blood-vessels, this 

 naturally must proceed from the heart, which, after having attained 

 a certain degree of development, begins its pumping action. In 

 accordance with the theory of Eemak and KoUiker, this can only 

 take place after the axial cells of those columns destined to the 

 formation of blood-vessels, have been loosened by the action of a 

 secreted fluid, and converted into blood corpuscles.* But this 

 theory is not in concert with my own observations regarding the 

 development of the first embryonic blood-vessels within the mem- 

 branes of the small human ovum, described at the beginning of this 

 essay. Neither in the blood-vessels of this ovum, nor in the 

 interior of those of older embryos, did I ever meet with blood cor- 

 puscles during the primary stage of the process of formation of 

 these vessels. I suppose, of course, that the mode of formation of 

 the primary blood-vessels — particularly of the smaller ones — in the 

 embryo itself, does not differ from that in the membranes and villi 

 of the ovum. Eegarding, therefore, the explanation given by the 

 last-named authors, as not entirely satisfactory, I shall take the 

 liberty of pointing to another mode, in which the blood of the 

 embryo might first be set in motion. 



In the smaller blood-vessels of human embryos from eight to 

 nine weeks old, the blood, accordmg to my own observations, makes 

 its way by the contractions of the heart, causing a continuous 

 advancing of the blood corpuscles into the capillaries newly formed, 

 but, as yet, not sufiiciently open. Now the circulation of the blood 

 within the provisional circulatory apparatus of the embryo, probably 

 takes place in the same manner, only in an inverse direction : pre- 

 supposed, however, that the heart and the blood-vessels of this early 

 period are not represented any more by solid masses or columns of 

 cells, but that, on the contrary, as I have reason to think, the 

 former already consists of rudimentary muscular fibrillae, and the 

 latter of deUcate, probably granular fibrillee of fibrous tissue with 

 numerous nuclei ; further, that a communication, by simple anasto- 

 mosis, between these vessels and the canals of the umbilical vesicle 

 is already estabhshed. The continuous generation of new blood 

 corpuscles, through the medium of the large hexagonal cells of the 

 umbilical vesicle, necessitates an accumulation of these bodies in the 

 interior of the follicles, as well as a distension of the walls of the 

 latter, and, in consequence, produces a force which must eventually 

 push the blood corpuscles from the follicles into the canals. But as 

 the generative process goes on without interruption, the canals also 

 become overfilled, and the blood corpuscles gradually penetrate into 

 the interior of the embryonic vessels with which the former com- 

 municate, and thus finally enter the interior of the heart. This 

 * Funke, 'Lelirbucli der Pli3'siologie,' 4. Auflage, B. II., p. 1141. 



F 2 



