THE BLOOD AND THE LYMPH. 719 



new formation of blood-corpuscles in the liver, with which the consider- 

 able size of, and the abundant supply of blood to, the embryonic liver, 

 is perfectly in accord, continues probably throughout the foetal life ; at 

 all events, I have found it in quite old embryos in jNIammalia, and also 

 in newly-born children, although it probably diminishes, pari passu, in 

 connection with the appearance of the ductus venosus (which, according 

 to Rathke, is a secondary formation) and its enlargement, because, 

 through it, a considerable portion of the blood from the umbilical vein 

 enters the circulation directly, and is diverted from the liver. 



The further development of the nucleated, spherical blood-cells of 

 the embryo, M'hicli have originated in either way, takes place in this 

 manner : the cells gradually, and either directly, or after they have 

 multiplied in the mode above described, become more and more flat- 

 tened, and even present slight excavations, wdiilst their nuclei mani- 

 festly diminish, and on the application of acetic acid, exhibit a great 

 tendency to disintegration. Ultimately, the nuclei disappear alto- 

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

 adult, of which they all soon assume the form, being at first somewhat 

 irregular. With respect to the period at which these non-nucleated 

 colored cells make their appearance, it must be remarked that in a 

 foetal Lamb, 3|^ lines long, I could perceive none of them, nor could 

 Paget in a human embryo, measuring 4 lines in the fourth week. In 

 foetal Lambs, 9 lines long, they were still very scanty, whilst in those 

 13 lines in length, they constituted by far the majority of the blood- 

 cells ; and in a human embryo, at three months, they formed, in the 

 hepatic blood \, and elsewhere about \-^ of the colored corpuscles. In 

 still older embryos, they preponderate greatly, so that in foetal Lambs 

 of 5-13 lines in length, the nucleated colored cells in the hepatic 

 blood constituted not more than \ or | of the blood-cells, and in the 

 rest of the blood, in the larger embryos, did not occur more abundantly 

 than the lymph-globules in the blood of the adult animal. At what 

 time, in the human embryo, the nucleated colored cells become more 

 rare and disappear, is not yet ascertained, although Paget saw them 

 still tolerably numerous in one instance in an embryo of five months. 

 The blood of the larger jMammalian embryos contains, not only in the 

 liver, but also elsewhere, besides the colored blood-globules, other color- 

 less cells in great number, and often as numerous as the colored, which 

 cells, there can perhaps be no doubt, are derived mainly from the liver, 

 in which, even in foetal Lambs 13 lines long, the colorless and slightly 

 colored, nucleated blood-cells constitute, perhaps, one-third of the 

 whole blood-corpuscles ; and in the latter periods of foetal life, are 

 probably also derived from the lymph. Whether these cells are meta- 

 morphosed into colored ones, is by no means determined, this much 

 only having been ascertained, that the transitionary forms between the 



