55 HOWELL. [Vol. IV. 



the liver from sections or from teased specimens, he finds 

 abundant proof for this in the transitional forms, which occur in 

 considerable numbers. With reference, however, to the origin 

 of these colorless cells, I cannot agree either with Neumann or 

 with Foa and Salvioli. In sections of liver in the later periods 

 of embryonic life one finds the nucleated red corpuscles and 

 their colorless predecessors lying between the rows of liver 

 cells, scattered irregularly, and without any very apparent 

 relationship to the other elements of the liver. But in the 

 earlier periods of embryonic life, — in the embryo, for instance, 

 2.5 cms. long, — sections of the liver show the origin of the 

 blood corpuscles quite distinctly. One sees in such sections 

 that the blood-forming cells are not scattered without order, 

 but are grouped into cords or strings lying between the col- 

 umns of true liver cells which are just beginning to show a 

 typical structure and arrangement. The cords of blood-forming 

 cells resemble those described by Wenckebach, Ziegler, etc., 

 in the germ layers of the young fish embryo, and here also 

 undoubtedly mark out future blood-vessels. One often sees the 

 solid mass of cells stop more or less suddenly while the channel 

 becomes filled with coagulated plasma containing here and 

 there fully developed red corpuscles with or without nuclei. 

 A drawing showing the appearance described is given in Figs. 

 13 and 14. I have obtained similar corJs of blood-forming 

 cells, evidently developing blood-vessels, in longitudinal sections 

 through the posterior limb of the same embryo, as shown in 

 Fig. 15, which indicates that though the production of red 

 corpuscles at this time is most active in the liver, it is also 

 going on in other parts of the body, probably wherever new 

 blood-vessels (veins) are forming. If we accept the theory 

 proposed by Kolliker, Wenckebach, and others, as to the 

 method of formation of the first blood corpuscles in the embryo, 

 then their production in later embryonic life is seen to follow 

 essentially the same plan. One might suppose, indeed, that 

 the cords of blood-forming cells in the young liver are directly 

 or indirectly derived from the original median mass of blood- 

 forming cells which first appears in the embryo, though I have 

 no observations at all which can be taken as evidence for such 

 an hypothesis. A similar method of origin of the red cor- 

 puscles in the marrow of birds during extra-uterine life has 



