47 
Since it is precisely in the red marrow, with its numerous 
lymphoid cells, that an accumulation of white or lymphoid 
cells is seen within the blood-vessels, the question at once 
arises—What is the connection between these facts? Are 
the lymphoid cells in the blood derived from those in the 
medullary tissue, or vice versd ? Neumann supposes that the 
lymphoidal cells of the blood are formed in the medullary 
tissue, and find their way into the vessels by a process of 
immigration similar to, but the converse of, that of emigration 
observed by Cohnheim; though, of course, not in this case 
susceptible of direct observation. The difference is, how- 
ever, very considerable between a cell finding its way out of 
a vessel in the direction of the blood pressure and into a 
vessel against the blood pressure. In order to make it pro- 
bable that the medullary cells do find their way into the 
vessels, it would be important to show they exhibit ameeboid 
movements. Neumann has observed that a small number 
only of the lymphoid cells pressed out of the medulla in a 
rapid investigation are without these movements, and con- 
cludes that it would be impossible to regard this small number 
as the only medullary cells; so that some of those showing 
amceboid movements must be medullary. 
More important support is given to the theory by various 
facts which point to a multiplication of medullary cells, since 
these, as they must go somewhere, Neumann concludes must 
find their way into the vessels. He also draws attention to 
the variations in size of the medullary cells, laying down the 
proposition that great differences in the size of similar 
elements in a tissue must depend upon processes of growth 
going on in them. Moreover, he draws attention to the 
myeloid cells or ‘myeloplaxes’ found in the marrow of bones, 
which cells, according to Kolliker, arise from a proliferative 
increase of the small medullary cells, and finally divide again 
into a large number of small cells. If, then, according to 
Neumann, such a multiplication of small cells does really take 
place as these facts point to, there is no other exit for them 
than into the vessels; the nutritive changes in so stable a 
substance as bone being far too small to need so copious a 
supply of organisable material. The hypothesis of an 
emigration of white cells from the capillaries into the medullary 
tissue, and a consequent accumulation of them here, is re- 
jected by Neumann on the ground that a tissue, like the 
medulla, enclosed in a hard shell of bone, is especially 
unfavorable for the emigration of cells, and that it is difficult 
to see what would become of the emigrated white cells, their 
return to the vessels being improbable, and their exit through 
