6 THE MATURATION OF THE EGG OF THE MOUSE. 



III. MATERIAL AND METHODS. 



The mice used at the beginning of this work were received from the 

 lot reared by Professor Castle and Dr. G. M. Allen in connection with 

 Dr. Allen's work on the Heredity of Coat Color in Mice. Some were 

 white and some were hybrids obtained by crossing wild gray mice (Mus 

 musculus) with the white variety bought of dealers. There were a few 

 white and hybrid individuals of less simple ancestry ; also black, choco- 

 late, and golden agouti (Allen, 1904). These served as a beginning for 

 the subsequent stock of 400 to 500 kept on hand for material during the 

 greater part of the past five years. 



As the vigor and fertility of the stock became lessened by inbreeding, 

 new white mice procured from several dealers in different parts of the 

 country and a few gray mice caught wild were introduced with bene- 

 ficial results. ' Thus the animals furnishing eggs for study were of cosmo- 

 politan ancestry. Besides the introduction of new blood, pains were 

 taken to mate as distantly related animals as possible in order to keep 

 up the standard of the stock. With the idea at first of finding out whether 

 there was any possible relation between the number of polar cells and the 

 coat-color inheritance, whites and hybrids were mated (giving whites and 

 hybrids in equal proportions) ; but on finding no such relation, hybrids 

 and whites were paired only for convenience in distinguishing sex. 



As a supplement to the account of the care of mice by Dr. Allen 

 (1904), whose methods the writer has in general used, the following may 

 be of value to those working with mice and rats. Fig. .4 (plate A) shows 

 a modification of the cage originally used in the Harvard Zoological 

 Laboratory. The improvement consists in making the lids a few inches 

 shorter and putting the hinges, not at the highest part of the cage, but 

 further down on the inclined surface. This arrangement greatly decreases 

 the danger of pinching under the lid frightened mice which have run up 

 the sides to the top, and, finding an opening, are trying to get out; it 

 also facilitates catching the mice in the upper corners. 



Since water left in open dishes soon becomes fouled, use was made of 

 the supply bottle shown near the corner of the left-hand cover in fig. .4 

 and in section at 5, fig. D (p. 9). One of these was put on each cage. 

 It consists of a 3-ounce, wide-mouth bottle fitted with a rubber stopper 

 pierced by a bent glass tube of about 6 mm. inside diameter. The tube 

 has its lower end bent just enough to prevent the escape of water when 

 undisturbed and is at the same time large enough and open enough to 

 allow air bubbles to ascend as the water is lapped out of the free end by 

 the mouse. This device, arranged as shown in fig. A, with the tube 

 projecting through the wire mesh into the cage, insures an easily acces- 

 sible supply of clean, fresh water. 



Mice thrive well on rich bread-and-milk, oats, and sunflower seed. 

 They find an occasional bit of lettuce a welcome addition. 



