53^ DEVELOPMENT OF THE BLOOD- |_ SECT - 222 - 



into the liver ; and I have also observed the formation within the 

 spleen of red nucleated cells, in the case of advanced embryoes, 

 and during the first year of independent life. Moreover, it appears 

 probable that at the earliest development of the liver, a portion of 

 these colourless formative blood-cells have their origin in the 

 process of formation of the vessels of this organ. The considerable 

 size and the vascularity of the embryonic liver completely har- 

 monises with this view of the new formation of blood-corpuscles 

 in that organ, and this process probably continues in the liver and 

 spleen throughout the whole period of embryonic life ; at least, I 

 have found it in quite advanced embryoes of mammalia, and also 

 in newly -born infants : it becomes, however, of less and less im- 

 portance, perhaps in connection with the appearance and further 

 development of the ductus venosus (which, according to Ratlike, 

 is a secondary formation), because, in this way, a considerable part 

 of the blood of the umbilical vein arrives directly in the circulation, 

 and is withdrawn from the liver. 



The further development of the nucleated, spherical blood-cells 

 of the embryo, in whatever manner formed, is effected by their 

 gradually becoming more and more flattened (either directly or 

 after they have multiplied in the manner stated above), and even 

 by their receiving slight excavations; their nuclei, meanwhile, 

 become decidedly smaller, and, on the addition of acetic acid, ex- 

 hibit a great tendency to break up. They ultimately disappear 

 completely, and the blood-cells become non-nucleated, like those of 

 the adult, and soon, also, become like them in form, instead of 

 remaining, as at first, of a somewhat irregular shape. As for the 

 period at which these non-nucleated, coloured cells make their ap- 

 pearance, it is to be remarked, that I did not observe them at all 

 in the embryo of the sheep, 3! lines long, nor were they seen by 

 Paget in a human embryo of the fourth week, which measured 

 4 lines in length. In embryoes of the sheep, 9 lines long, they 

 were still extremely scanty; but in those which had attained a 

 length of 13 lines, they constituted the great majority of the 

 blood-cells. In a human embryo at the third month, these 

 coloured corpuscles numbered about one-fourth of the whole in 

 the blood of the liver; while in the blood of other parts, they 

 constituted from one-sixth to one-eighth of the whole. In still 

 older embryoes the non-nucleated cells greatly preponderate, so 

 that in sheep-embryoes of 5 to 13 inches in length, the nucleated 

 coloured cells in the hepatic blood constitute only one-fourth or two- 

 fifths of the blood-cells ; and in still larger embryoes, in the general 



