152 ANIMAL PHYSIOLOGY. 
There is no doubt that the greater number of the non-nucle- 
ated corpuscles are derived from the nucleated forms. 
The post-embryonic development of colored corpuscles is 
naturally less understood from the greater difficulties attend- 
Fia. 149, 
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Fie. 151. Fie. 152, 
Fig. 148.—Cell elements of red marrow. a, large granular marrow cells; b, smaller, more 
vesicular cells ; c, free nuclei, or small lymphoid cells, some of which may be even sur- 
rounded with a delicate rim of protoplasm; d, nucleated red corpuscles of the bone 
marrow. 
Fic. 149.—Nucleated red cells of marrow, illustrating mode of development into the ordinary 
non-nucleated red corpuscles. a, common forms of the colored nucleated cells of red mar- 
row ; b, 1,2,3, gradual disappearance of the nucleus ; c, large non-nucleated red corpuscle 
resembling 2 and 3 of b, in all respects save in the absence of any trace of nucleus. 
Fic. 150.—Nucleated red corpuscles, illustrating the migration of the nucleus from the cell, a 
process not unfrequently seen.in the red marrow. 
Fig. 151.—Blood of embryo of four months. a, 1, 2,3, 4, nucleated red corpuscl In 4 the 
same granular disintegrated appearance of the nucleus as is noted in marrow cells. b, 1, 
microcyte ; 2, megalocyte ; 3, ordinary red corpuscle. 
Fie. 152.—From spleen. 1, blood-plaques, colorless and varying a little in size; 2, two micro- 
cytes of adeep-red color ; 3, two ordinary red corpuscles ; 4, a solid, translucent, lymphoid 
cell or free nucleus. (Figs. 148-152, after Osler.) 
ing its investigation. The following may be regarded as a 
summary of the chief facts or rather opinions on this subject: 
1. From the colorless cells; though, whether the nucleus 
disappears, or remains to form the chief part of the cell and 
become pigmented, is undetermined. 
2. From peculiar cells of the red marrow of the bones (head, 
trunk, etc.), though there is also some doubt as to whether the 
