8 THE MATURATION OF THE EGG OF THE MOUSE. 



fairly constant habit of mice to take food or water at frequent intervals. 

 The food is placed on a stationary shelf in the cage, whereas the nest 

 and the floor of the cage are made independent of the rest of the cage 

 and of each other. By making the movements of the delicately poised 

 nest and floor self-recording, the activities of the mouse can be deter- 

 mined. The change in the record after parturition is due to the increase 

 in the weight of the nest depending on the presence of the young mice 

 in the nest even when the mother is away. The apparatus is constructed 

 on the principle of a simple balance, the movements of which are re- 

 corded on a chronograph drum revolving once in 12 hours. The parts 

 shown in fig. D at 1 and 2 constitute a unit and accommodate one mouse. 

 The apparatus as finally perfected, fig. C, has a capacity of four mice, all 

 the records being made simultaneously on the same chronographic drum. 

 The essentials of each unit are shown in fig. D; 1 is a diagrammatic 

 side view, and 2 is an end view. Each unit consists of a box, fixed in 

 position, but having a movable floor composed of two parts, each of 

 which is suspended independently of the other and may move in a 

 vertical direction. The box (B), about 15X12X10 inches, rests upon 

 supports as seen in figs. B and C. Each box has either the top or side 

 made of wire netting having quarter-inch meshes and is provided with 

 a door (D) at one end or on the top (see lower box on right side and 

 upper box on left side, fig. C). The floor is of thin, light wood made in 

 two separate parts — a central part, the nest-floor (fig. D, 1 and 2, NF), 

 supporting the nest (N), and a marginal part, the main floor (MF). 

 The two parts of the floor are suspended from the ends of two levers or 

 balance arms (NL, FL), the opposite ends of which terminate in pointers 

 (NP, FP) in contact with the revolving drum of a chronograph (CR). 

 The levers are supported on pivot fulcrums at O O, and the pointers are 

 made of very thin spring-brass so pointed and bent as to scratch the 

 smoked paper enveloping the drum. The suspension of the floors is 

 effected by means of thin strips of wood the upper ends of which are 

 attached to the ends of cross-beams. Each cross-beam in turn rests on 

 the end of its lever by means of a glass-and-steel bearing. To the under 

 side of the middle of each cross-beam is attached a piece of glass (G, fig. 

 D, 1, 2, 3, 4), which rests on a steel knife-edge (E) secured to the end of 

 the lever (NL or FL) . Slipping of the glass on the steel edge is prevented 

 by making a slot (fig. D, 3 and 4, SL) in each of the two pieces of sheet 

 zinc (Z) with which the glass (G) is held in place on the under side of 

 the cross-beam, the knife-edge (E) occupying the slot. To all the edges 

 of each floor are fastened strips of light tin (T). These prevent the 

 mouse from easily gnawing out and also keep in place the nest (N, which 

 is an inverted strawberry basket) and the sawdust with which the main 

 floor is sprinkled. To the floors are further attached light wood strips 

 (S S, provided with metal ends for reducing friction), which keep the 

 floors from touching each other or the box. The floors and attached 



