250 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. Iam 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 
