710 ^ J. Marion Read 



be normal m every way. As far as possible adult mothers were 

 chosen, and so no litters consisting of only one are included J ). The 

 cases designated as Xj and Kj were first litters, however, containig 

 three and two respectively. No cases were included in which the 

 mother became pregnant immediately after parturition, thus nursing- 

 one litter while carrying another. 



All of my animals were weighed every second day, so there is 

 a complete record of the weights of all for the past fifteen months. 

 Table I, column A, gives the weights of the mothers at copulation. 

 Except in the cases noted under remarks the weight is that actu- 

 ally observed, recorded within twenty- four hours before or after co- 

 pulation. In the four cases noted under remarks the weight in 

 column A was raised to an average of the weights for a week or 

 two preceding. In most cases the weight at copulation was very 

 close to the average weight. 



For the sake of comparison and as an aid to correct valuation 

 of the results, I am publishing two tables. Table I gives the weights 

 as they were actually recorded and Table II shows these weights 

 again with six of them corrected as explained under > remarks . 



It may be noted here while discussing the weights of adult 

 guinea-pigs and the corrections I have deemed it advisable to make, 

 that these animals display considerable variation in weight from day 

 to day. In an adult guinea-pig of 650 850 gins, a variation of ten 

 or twenty grams is not exceptional. MiNOT 2 ) says, In all the 

 weighings there is necessarily an error . A positive error because 

 the digestive tract, particularly the wide caecum, contains always 

 considerable quantities of undigested material; moreover the bladder 

 may hold a greater or less quantity of urine. A negative error be- 

 cause every illness, even a very slight indisposition and every in- 

 jury such as a bite for instance, causes a greater or less loss of 

 weight. The quantitative value of these errors is presumably not 

 very great; they probably counterbalance one another to a certain 

 extent in the averages which may be accepted as approximately 

 accurate. Guinea-pigs are very sensitive animals and a very slight 

 disturbance will cause considerable change in weight. At a recent 

 weighing almost all of my animals had lost heavily due to the fact 

 that they had been moved the day before from one animal house 



*) Immature mothers usually bear one in a litter. 

 2 ) C. S. MINOT, loc. cit. 



