344 SPECIAL HISTOLOGY. 



of them, nor could Paget in a human embryo, measuring 4'", 

 in the fourth week. In foetal Lambs, 9'" long, they were still 

 very scanty, whilst in those 13'" 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 else- 

 where about I — i of the coloured corpuscles. In still older 

 embryos, they preponderate greatly, so that in foetal Lambs of 

 5 — 13"' in length, the nucleated coloured 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 

 coloured 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 Mammalian embryos contains, not only in the liver, but 

 also elsewhere, besides the coloured blood-globules, other 

 colourless cells in great number, and often as numerous as the 

 coloured, which cells, there can perhaps be no doubt, are 

 derived mainly from the liver, in which, even in foetal Lambs 

 13'" long, the colourless and slightly coloured, 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 metamor- 

 phosed into coloured ones, is by no means determined, this 

 much only having been ascertained, that the transitionary 

 forms between the two, so numerous in the hepatic blood, are 

 wholly wanting in the rest of that fluid. 



The origination of the blood-globules after birth and in the 

 adult, notwithstanding the great pains specially devoted to this 

 point, still remains one of the most obscure parts in the history 

 of the blood-cells ; in my opinion, however, the notion which 

 assumes that the red blood-cells proceed from the smaller chyle- 

 corpuscles, which lose their nuclei, become flattened, and have 

 hematin produced in them, is the one most deserving of credit. 

 These cells are about of the same size as the blood-globules, 

 or even rather smaller, have the same kind of membrane as the 

 latter, are flattened, and not unfrequently of a faint yellow 

 colour, and consequently may, as we see in the colourless 

 blood-cells of the embryo, pass without any considerable change 



