294 W. B. KIRKHAM AND H. S. BURR 



agree with Miller ('11) and King ('13) that mother rats, unless 

 they are in an unhealthy condition, or have been frightened in 

 some way, rarely if ever kill or maim their young. 



Albino rats give birth to young in all seasons of the year, 

 but it is only from April to October that ovulation as a rule 

 occurs within 48 hours after parturition; during the remaining 

 months they are apt to skip oestrus cycles, ovulation not occur- 

 ring until some three weeks after parturition. 



The senior author showed in 1910 that the albino rat o\ti- 

 lates regardless of whether pairing has previouslv taken place, 

 and when males are continuously present copulation may occur 

 before the ripest eggs in the ovaries have formed the first polar 

 spindles. On several different occasions we have seen the actual 

 pairing. It differs markedly from the condition described by 

 Sobotta ('95) for the mouse, since the male albino rat is not 

 prostrated by the sexual act, but walks slowly away. When 

 a previously isolated female who is in heat is placed in a cage 

 wdth several males they will all pair with her in rapid succession. 



The period of gestation in the albino rat is twenty-two days 

 when the female is not nursing a previous litter, in which event 

 the period may be lengthened as found by King ('13). The 

 litters varied in number from four to twelve and the birth usu- 

 ally took place in the late afternoon or the early evening, although 

 probably it may occur at any hour of the day, since we have 

 observed it at noon. The process of parturition is briefly as 

 follows : The female in order to aid in the expulsion of the foetus 

 flattens herself against the bottom of the cage while a series of 

 wave-like muscular movements pass posteriorly along the body 

 starting just behind the shoulder. As soon as the young rat is 

 free from her body, the female rises up on her haunches, seizes 

 in her forepaws the button-like placenta? which is still attached 

 to the offspring by the umbilical cord, and devours first it and 

 then the cord, cutting off the latter as close to the body of the 

 young animal as she can get with her teeth. The female then 

 again flattens herself out against the bottom of the cage pre- 

 paratory to the appearance of the next young rat. The process 

 is repeated until all have been brought forth. Then, and not 



