File:LongMark1911 figD.jpg

Fig D

In order to increase accuracy in observation and to save much time during both day and night, the apparatus illustrated in figs. B, C (plate A), and D was planned and made by the junior writer. Its purpose was to serve in recording automatically the approximate time of the birth of litters. In this apparatus advantage has been taken of the 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 determined. 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 recorded on a chronograph drum revolving once in 12 hours. The parts shown in fig. D at i 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; i 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, i 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, i, 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 parts are counterbalanced by the weights (W W) , which may be so adjusted that the floors move up and down at a very light touch. The extent and place of the excursion of the levers are controlled by check blocks, shown in plate A, figs. B and C, attached to the outside of the chronograph box. The feed dish (FD) is on a little shelf attached to the inside of the box, and thus independent of the movable parts, as is also the water bottle.

In use, the weights are so adjusted that the empty nest (N) with its floor (NF) is raised to its upper limit, but may be depressed by a weight of only 2 to 3 grams; the main floor (MF), on the other hand, requires about 10 grams to depress it.

When, therefore, there is no mouse present, both parts of the floor (NF and MF) are up, and the pointers (NP FP, fig. E) are down. If, under these conditions, the chronograph drum is set in motion, the two pointers will inscribe lines in the position of the lines between a and b, fig. E. A pregnant female placed on the floor (MF) causes its pointer (FP) to go up, as at b. As long as she remains on the floor the record is like that between b and c. When she enters the nest, the nest pointer (NP) goes up and the floor pointer (FP) down, the record being that between c and d. When she leaves the nest and goes directly to the food, the record becomes that between d and e, as at first. The record at e shows that she again enters the nest, but on her way jumps to the main floor (vertical mark on line FP). If, before again making an exit (as she must for water and food), she gives birth to a litter of little ones, on the one hand her weight will still be sufficient to depress the floor (MF) , as at /, and on the other, the young will be heavy enough to keep the nest down, so, that no matter how often she goes in and out, the nest pointer (NP) will make an unbroken line, the floor pointer alone making vertical marks.

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Reference

Long JA. and Mark EL. The maturation of the egg of the mouse. (1911) Carnegie Inst. of Washington Pub. No. 147. 77.