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ZENKER’S killing fluid is used rather extensively by medical men 
in the study of human histology and gives uniformly good results 
with such tissue. It seems, however, to be almost unknown to 
zoologists and I feel justified from my experience with this reagent 
in recommending it for use in fixing animal tissues. I have used it 
in the study of various tissues of birds and mammals and find it 
very reliable. 
Several staining methods were employed. FLEMMING’s orange 
method differentiated the achromatic and chromatic material sharply. 
HEIDENHAIN’sS iron-haematoxylin gave clear definition of all chromatic 
substance including nucleolus and centrosome. 
Upon an examination of the sections, the absence of karyokinetic 
stages was most striking. The first three slides which were examined 
presented not so much as a single example of a cell in process of 
division. It was supposed that these sections had come from a resting 
zone of the testis. The whole organ was, therefore, sectioned and in 
the great number of sections thus obtained, not more than twenty 
cells were found in mitotic condition. 
The state of preservation of the material was evidently good and 
the absence of mitosis can, therefore, not be charged to the account 
of poor microscopic technique. The few cases of mitosis observed 
were in spermatocytes of the first order. One could easily distinguish 
spermatogonia, spermatocytes of the first and second order, spermatids, 
and numerous nearly mature spermatozoa. The number of the latter 
to be seen was very large and precludes the assumption that the 
testis was functionally impaired by age or by the hernia. In the 
opinion of the writer, this condition merely indicates that all the 
various processes in the spermatogenic series are not necessarily to 
be observed as taking place at the same time. I can see no reason 
why there might not become established in the testis periods of 
cellular activity alternating with periods of cellular rest. This could 
not be taken to mean absolute rest, but merely a relatively lower 
degree of activity. It appears moreover that one process may be 
completely interrupted while others are going on actively. 
Thus in my material mitosis was practically at a standstill while 
the growth of the spermatocytes, their anaphase preparations for 
division, and the metamorphosis of the spermatids were in active 
progress. Evidences of such periodicity are very slight in insects and 
this is just what we should expect from the nature of the case. In 
mammals, however, there are good and obvious reasons to account 
for the establishment of spermatogenic periods. 
