THE STORY OF THE RED BLOOD CORPUSCLES. 111 
structure thus formed becomes differentiated into an outer 
envelope which forms the wall of an embryonic blood 
vessel, and an inner mass of small nucleated cells which 
are the young blood corpuscles. Even in mammals al) 
the blood corpuscles are nucleated during the early stage 
of development, and even after birth some nucleated red 
cells may be found in the general circulation, but in a 
short time only the characteristic non-nucleated, biconcave 
dises are to be seen. 
What relation do these discs bear to the nucleated 
corpuscles of the embryo? This is a question which has 
led to a great deal of debate. It was at one time believed 
by many of the best observers that the ordinary red cor- 
puscles were derived from the nuclei of cells which had 
lost their protoplasm and had become pigmented. There 
can, however, be no doubt that this view is incorrect, and 
that the red corpuscles of the adult really represent the 
protoplasmic portion of nucleated corpuscles from which 
the nucleus has disappeared. 
The question of the origin of the red corpuscles after 
birth was very obscure until comparatively recently. The 
spleen and the lymphatic glands were supposed to play an 
important part in the formation of red corpuscles, which 
were described as arising either from the cells of the 
leucocyte class, which are so numerous in these organs, or in 
the interior of large blood-forming cells, as they do in the 
embryo. At the present day it is regarded as quite certain 
that red corpuscles are not derived from leucocytes, and 
it is very doubtful whether an intracellular formation of the 
red cells ever occurs in extra-uterine life. Much more 
probably, all the ordinary red corpuscles arise directly from 
nucleated red cells. 
As long ago as 1868 Neumann and Bizzozero inde- 
pendently discovered nucleated red corpuscles in the bone- 
marrow of mammals. This discovery naturally excited a 
great deal of interest, and led to much investigation of the 
structure of marrow. The earlier researches were much 
hampered by difficulties of technique, and it was not until 
the paraffin method of cutting sections was introduced that 
satisfactory preparations could be obtained, showing the 
