6 G. CARL HUBER 



rats. Dr. J. M. Stotsenburg, to whose experience and care- 

 ful records I am greatly indebted for the trustworthiness of 

 the material collected, made use largely of females who had 

 born one litter. Pairing was seldom attempted a few hours 

 post partem, as was done by Sobotta and Burckhard, but usually 

 about 30 days after the birth of a litter, which may have been 

 nursed or otherwise disposed of. The great majority of females 

 used in pairing were at the time free from 'domestic cares.' 

 The females employed were kept in separate cages for some 

 time before giving birth to young and until the time of mating. 

 About 30 days after the birth of a litter, a male was placed 

 in the cage with the female. If the female was in heat, copu- 

 lation usually took place soon after. The male was left with 

 the female for an hour to an hour and a half, during which time 

 several pairings would occur, and at the end of which time the 

 female would try to hide from the male, climb the side of the 

 cage and defy him with her teeth. The male albino rat is not 

 prostrated by the sexual act, the same male serving for several 

 successive copulations. In case the female was not in heat, this 

 soon became evident and the male removed, to be again placed 

 into her cage 24 or 48 hours later. The time when the. copu- 

 lation was first observed was noted on the card attached to the 

 cage and gave the time from which the age of the embryo or 

 respective stage was reckoned. The time given is, therefore, 

 that of 'insemination,' a term which Long and Mark have intro- 

 duced to indicate "the introduction of the male sexual elements 

 into the genital tracts of the female by the act of coitus or other- 

 wise." This time could be accurately noted, while 'semina- 

 tion' which "applies to the access of the spermatozoa to the 

 eggs in the oviducts, the coming into contact of the male and 

 female reproductive cells" can not be accurately timed. The 

 success attained in pairing albino rats as above stated, obviated 

 the necessity of depending upon chance material or resorting 

 to 'artificial insemination' as described for the mouse by Long 

 and Mark. I am at loss to understand why Widakowich should 

 regard the age determinations of Sobotta and Melissinos (mouse 

 embryos) more accurate than his own, reckoned from the time 



