548 EZRA ALLEN 
MATERIAL AND TECHNIQUE 
I am indebted to The Wistar Institute of Anatomy and Biology 
for the albino rats used in the study and for the use of its labora- 
tory equipment. I am indebted to Dr. Stotsenbtirg, of the Insti- 
tute, for a supply of rats as needed and for courteous attention; 
to Dr. H. D. King for a method of imbedding and staining; to 
Dr. S. Hatai for valuable suggestions. To Dr. H. H. Donaldson 
I am especially indebted for constant aid and criticism. 
My method of work was in general as follows. Healthy rats 
of the following ages were used: 1, 4, 6, 7, 12, 15, 18, 20, 30, 52, 70, 
about 120 days old, and one about two years old. In all, some 
twenty-five different animals were studied. Record was made of 
the weight, body-length and sex. After chloroforming the animal, 
the brain and cord were quickly removed, fixed in Carnoy’s fluid 
and imbedded, the younger specimens in paraffin the older in 
eelloidin and paraffin. Frontal sections were then cut at 8 micra 
and stained with either iron-alum-haematoxylin or thionin, fol- 
lowed by eosin or erythrosin. Thionin was found to differentiate 
the mitotic figures with sufficient clearness for this:study. By 
using the mechanical stage, the sections were thoroughly explored 
with a magnification sufficient to detect mitoses. Only those 
cells which showed mitotic figures clearly were enumerated; the 
earlier stages of prophase, being more difficult to identify, were 
not considered. The Zeiss 4 mm. objective and No. 4 eye- 
piece usually sufficed; in doubtful cases the 2 mm. oil immersion 
objective and eye-pieces higher than No. 4 were employed. 
Two methods of record were used. With the younger material, 
diagrams representing the sections studied were outlined and on 
these the approximate location of the dividing cells was indicated, 
the number / being employed for the cells in section No. 1, the 
number 2 for those in section No. 2, etc. (figs. 1 to 4). When 
the examination of a series was complete, this method showed 
graphically the distribution of the dividing cells in that portion’ 
of tissue both in cross section and longitudinally. The other 
method, usually employed for the older material, was to tally the 
cells as discovered opposite the number of the section, regional 
location being indicated by columns if desired. 
