as Enemies of Mankind. 



II 



3^- oz.) in i?. norvegicus ; but the average weight of apparently 

 adult specimens is certainly not less than 140 grammes (about 

 5 oz.) in B, rattus, and 250 grammes (about 9 oz.) in B. norvegicus, 

 " Apparently adult rats " are those in which the coat and colour 

 are similar to those of old animals, in which the hind-foot measure- 

 ment amounts to less than 25 per cent, of the head and body 

 length, and in which, being females, the vagina is perforate ; such 

 rats are certainly at least four months old ; growth continues until 

 they are eighteen months old, and perhaps does not cease then. 

 De risle, experimenting with B. rattus, found it to be sexually 

 mature when less than three months old ; and F. Buckland records 

 that a female B. yiorvegicus in captivity bore a litter of eleven 

 young when only eight weeks old, so that she must have been 

 impregnated at the age of five weeks. 



Although rats breed in every month of the year, pregnant and 

 nursing females are more common between the months of January 

 and June than at other times. Full-grown males, in good health 

 and normal conditions, appear to be capable of pairing at all times. 

 The females can only pair at certain times ; they have a long 

 sexual season, extending (for any particular female) possibly through 

 nine months of the year. During this season they experience 

 desire, or come on " heat," in the absence of the male, at intervals 

 of about ten days, but "heat" lasts for a few hours only; if not 

 satisfied it subsides, and the female cannot then be impregnated 

 until her next heat." The period of gestation is normally about 

 twenty- one days ; but if the female be already nursing at the time 

 of impregnation, the development of the embryos may be retarded, 

 and the period is prolonged by another ten days or so. During 

 the sexual season some evidence of the regular recurrence of 

 "heat" is shown even by the pregnant female; and parturition, 

 in the season, is immediately followed by " heat," so that impreg- 

 nation is renewed within a few hours of the birth of a litter. 



Lataste, to whom primarily we owe a very large share of our 

 knowledge of the breeding habits of Muriclae, found that after 

 effective coitus the vagina is plugged by a stopper, the purpose of 

 which apparently is to prevent the escape of semen from the 

 female before impregnation has taken place. This vaginal stopper 

 is a joint production of the sexes, its larger, central and quickly 

 coagulating portion being furnished by the male. The stopper re- 

 mains in place for some hours, and is then expelled from the vagina. 



It may be mentioned here as a point of practical importance 



