GESTATION OF THE ALBINO RAT. 385 



that have been observed in the various mammals mentioned, 

 although I have not been able to find any records that exceed 

 them. 



2. CASES OF SUPERFECUNDATION. 



As there is much confusion in literature regarding the use 

 of the words superfecundation and superfoetation, it seems advis- 

 able to define these terms as they are employed in this paper. 

 Superfecundation is used in cases where two or more ova belong- 

 ing to the same period of ovulation were fertilized by successive 

 matings. Superfoetation, on the other hand, is applied to cases 

 in which ovulation, followed by copulation, occurred during 

 pregnancy and led to the simultaneous development in the 

 uterus of two sets of ova belonging to different periods of ovula- 

 tion. 



Although no observations have been recorded regarding the 

 time required for parturition by the rat it is generally assumed 

 that all members of a given litter are born within five or six 

 hours, as is the case in the mouse according to the investigations 

 of Long and Mark and of Daniels ('12). While this rule doubt- 

 less holds good in the great majority of cases there are occasional 

 exceptions, as is shown by the following instance. 



Female 8B 5 gave birth to a litter on April 8, 1912. It became 

 necessary to move the mother and young the day after the litter 

 was born. Six young rats were transferred to the new nest and 

 all of them were apparently of about the same size and weight. 

 Three days later the new nest was examined to see if the trans- 

 ference had caused the female to destroy her offspring. This 

 time eight individuals were found in the litter; two of them were 

 decidedly smaller than the rest and they had the appearance of 

 having been born but a very short time. In this instance there 

 was an interval of at least two days between the birth of the first 

 six members of the litter and of the last two. 



On examining litters of albino rats that are from six to fourteen 

 days old one occasionally finds that one or more of the young 

 rats are decidedly smaller and more immature than the others. 

 Such small individuals have been considered by many investi- 

 gators as runts, and their size has been attributed to the fact 

 that, being constitutionally weaker, they were unable to obtain 



