292 W. B. KIKKHAM AND H. S. BURR 



mouse egg. These investigators never saw a definite first polar 

 body associated with an egg in the tube. 



Newton Miller's paper ('11) on reproduction in the brown rat 

 is based solely upon observations of the living animals. He 

 found that both sexes become sexually mature ''at least by the 

 end of the fourth month," that the litters contain from six to 

 nineteen young apiece, and that these animals breed the year, 

 round. 



Mark and Long ('12) devote most of their contribution to an 

 extended description of the elaborate warm chamber they have 

 devised for the study of living mammalian eggs. When it comes 

 to the results obtained with their apparatus they have, at pres- 

 ent, little to say. Living eggs of rats and mice obtained in a 

 manner similar to that described by one of us (Kirkham '07) 

 were placed on the stage of the microscope in the warm chamber 

 and spermatozoa added, the mouse eggs underwent no change, 

 but the rat eggs within five minutes to two hours began the 

 formation of the second polar cell. Cleavage has never been 

 observed, and after twelve hours the eggs begin to degenerate. 



The latest contribution to the literature on the subject of 

 rat breeding is by Helen Dean King ('13) who records for the 

 albino rat somewhat the same phenomena previously observed 

 by Daniels ('10) in mice. The normal period of gestation for 

 the albino rat, according to Miss King, is twenty-one to twenty- 

 three days. If six or more young are being carried while a pre- 

 vious litter of five or less are still suckling the period of gesta- 

 tion may be prolonged, while if more than six young are suckling 

 the period is always prolonged, regardless of the number being 

 carried. Unlike the mouse, the albino rat appears not to exhibit 

 any exact relation between the number of young either suckling 

 or borne and the extent of prolongation of the gestation period. 

 This paper also contains evidence that the eggs of a given oestrus 

 cycle in the albino rat may be discharged from the ovaries in 

 two sets, with an interval of two to three days, and also that 

 in very rare instances this interval may be extended to two 

 weeks. Miss King would like to interpret the latter cases as 

 instances of a distinct oestrus cycle occurring during pregnancy. 



