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THE AMERICAN NATURALIST [Vol. XL VI 



at which the young should be weaned, no definite change, 

 except increase in size, is shown. 



With the period of sexual maturity we may count the 

 cycle of development complete. This does not imply, how- 

 ever, that growth ceases at this time. The time at which 

 mice reach sexual maturity varies greatly. While I have- 

 had some mice to pair at six weeks, this is rather unusual. 

 It has been my experience that under ordinary circum- 

 stances both the males and females reach sexual maturity 

 in the second or third month. From this time on for the 

 next few months the mice are in the prime of the repro- 

 ductive period, beyond which, at the end of ten to twelve- 

 months of age, activity diminishes and, for purposes of 

 breeding, the mouse is of little further service. 



V. Peactical Stjmmaey 

 The ease with which white mice can be handled makes 

 them in many cases preferable for experimentation to- 

 other and larger rodents, but their usefulness has been 

 greatly curtailed because of a wide-spread notion that; 

 they are difficult to rear under the conditions of the 

 laboratory. 



The purpose of this paper is to give an intensive* 

 method by which I have been able to rear then in abun- 

 dance. By "intensive" I mean that relatively few mice 

 are kept from which to breed, and that these are kept 

 under conditions which insure productivity. 



Especially detrimental to intensive breeding are 

 parasitism and marked fluctuations in temperature. Hot 

 air heat, the temperature ranging from 20° to 25° C, has 

 proved most satisfactory. Heat from an oil stove when 

 continued for a considerable length of time proved un- 

 satisfactory. 



Mice which are badly parasitized are useless for breed- 

 ing purposes. Various kinds of sprays and powders are 

 used to rid them of the parasites. Some of these, how- 

 ever, I have used with disastrous results. Parasitism 

 may be best prevented by using a case that is readily 

 cleaned. My cases are washed weekly with hot water to 



