ovary could not be followed, as females are not slaughtered at the age of fertility. 

 In the male, however, the testes and the epididymis show the following changes: 



The peak in the weight of the testes in autumn does not have the same signi- 

 ficance. The peak of 96 g in February- March is connected with the rutting season, 

 whilst that of 91 g in August- October is due to heavy hydration. During December- 

 March the diameter of the tubules is 183^ with 5-6 layers of germ cells in various 

 stages of differentiation with abundant mitoses. Many sperms are seen between the 

 Sartoli cells, their tails directed towards the lumen. The walls of the tubules are 

 highly acidophilous. The space between them is narrow, and their walls often touch 

 those of neighbouring tubules. The intersitial tissue is compressed, and its nuclei 

 are stained a dark colour by haematoxylin. 



From March onwards, degeneration sets in; more and more of the tubules are 

 devoid of sperms and vacuolization becomes conspicuous. In May this degeneration 

 is well advanced; a few wall layers only are seen in the tubules and these are often 

 reduced to one. Sperms are almost absent, as are mitoses. The diameter of the 

 tubules is reduced to l3l(Ji, and the interstitial tissue is loose. In October, rejuve- 

 nation of the germ cells begins simultaneously with the disappearance of the dege- 

 nerated cells. The diameter of the tubules slowly increases, and an ever increasing 

 number of sperms is observed. From November to January, rejuvenation progresses, 

 reaching its annual peak in February. Observations on the epididymis and the 

 presence of sperms within its lumen agree with the seasonal trend of the testes. 

 The thyroid shows colloidal accumulation in extending lumina from July to Novem- 

 ber. Until February the cells remain flat, the nuclei are at rest and no further secre- 

 tion is observed. During February the cells of the glands become columnar and their 

 nuclei are reduced. From March and April onward, a slow preparatory accumulation 

 of colloidal secretion into the lumen begins. The seasonal peak of the thyroid close- 

 ly follows that of the testes. 



The reproductive season of many species of small rodents in the desert also 

 seems to be concentrated in the short climatic spring, although prolonged breedings 

 of Meriones, Acomys, etc. have shown that under suitable environmental conditions 

 they are able to continue reproduction throughout the year. The limiting factor 

 seems to oe the composition of the food. We have shown for Microtus that in Israel, 

 certain factors in green plants may induce a duplication of fertility by raising the 

 number of eggs per ovulation, as well as by shortening the interval between two 

 consecutive pregnancies. Microtus is apparently unable to maintain fertility on a 



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