VII 



THE BREEDING OF MICE IN LABORATORIES 



Mating Habits. The oestrous cycle (2) of the mouse lasts 

 from three to four days. In only a few hours within this 

 cycle will the female permit the amours of a male; at other 

 times she avoids his attentions by slipping quietly into the 

 nest or by climbing up the side of the cage. 



During the receptive period her reactions to courtship 

 change completely. When her suitor approaches, she often 

 rears upon her haunches, throws her paws up in a defensive 

 attitude, closes her eyes, and gives a characteristic short 

 squeak. The male nervously licks her face, sniffs at her 

 genitals, and attempts to mount. The female often runs 

 away into the nest or up the side of her cage only to return to 

 the former trysting place. There are numerous unsuccessful 

 attempts at copulation before the completed act takes place. 



The secretions of the male form a soft vaginal plug which 

 quickly hardens, cementing the vagina shut and thus pre- 

 venting escape of the sperm until after fertilization (125) of 

 the eggs is accomplished. The vaginal plug is usually lost 

 within twenty-four hours. 



Uterine Development (129, 81). The sperm ascend the geni- 

 tal tracts within a few hours after copulation and fertilize 

 the eggs as they are shed from the follicles before they enter 

 the uterus. The fertilized eggs descend into the uterus where 

 they implant about the fifth day. At this time development 

 has progressed to about the primitive streak stage. About 

 the eighth day, at a stage corresponding to that of the three- 

 day chick, all the foetal organs are laid down. The cerebral 

 ganglia are growing out, and the limb buds are forming, 

 while the heart has long been pumping blood through the 

 foetal arches. The retina has invaginated and the lens is 

 formed. 



47 



