THE BLOOD AND THE LYMPH. 343 



may be found in the circumstance that, at this time, all the 

 blood from the umbilical vein, which supplies the embryo 

 with new organisable materials, first flows into the liver, instead 

 of entering the general circulation as before. In proportion to 

 the extent in which this cell-formation in the hepatic vessels is 

 carried on, does the self-multiplication of the blood-corpuscles 

 become less and less considerable; and instead of it, a new 

 formation of free colourless nucleated cells, having a mean size 

 of 0-003 — 0-004'", and an extreme diameter of 0*0015 — 

 0-006'", is observed to take place in the blood and imme- 

 diately around nuclei, which also occur in the free condition ; 

 and which cells afterwards, still for the most part in the liver, 

 are transformed, by the development of colouring matter in 

 their contents, into coloured, nucleated blood-cells, either imme- 

 diately or after they have multiplied in a similar way to that 

 which the coloured corpuscles had previously followed. This new 

 formation of blood-corpuscles in the liver, with which the con- 

 siderable 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 Mammalia, and also in newly-born 

 children, although it probably diminishes, pari passu, in con- 

 nexion with the appearance of the ductus venosus (which, 

 according to Rathke, is a secondary formation) and its enlarge- 

 ment, 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, which 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 flattened, and even present slight ex- 

 cavations, whilst their nuclei manifestly diminish, and on the 

 application of acetic acid, exhibit a great tendency to dis- 

 integration. Ultimately, the nuclei disappear altogether, 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 coloured cells make their appearance, it must be re- 

 marked that in a foetal Lamb, 3£'" long, I could perceive none 



