48 LABORATORY MOUSE 



The young are born usually between the nineteenth and 

 twenty-first days after mating {103, 29). In Japan wild 

 house mice and those of the fancy are often termed "twenty- 

 one-day mice" on account of the length of the gestation 

 period. 



Birth. Just prior to parturition the female may be ob- 

 served to become restless. During this period she may con- 

 struct an elaborate nest by winding strands of nesting tissue 

 to form a ball, tucking in loose ends here and there. 



At the birth of young, the reactions of the mother seem 

 to vary greatly with such factors as her natural temperament, 

 health, company, and the degree of seclusion provided by her 

 cage. Some mice retire to the nest for parturition while 

 others continue their routine reactions about the cage. 



Reaction to the new-born young may vary from almost 

 completely ignoring them where they happen to drop out- 

 side the nest, to birth within the nest followed by careful 

 washing and cuddling. 



Four or five minutes of uterine contraction may be re- 

 quired to give birth to a mouseling, followed within a minute 

 by the expulsion of the placenta and perhaps portions of 

 the fcetal membranes. The female chews and occasionally 

 devours the placenta. 



The new-born mouse is a naked little creature about two 

 and a half centimeters long with well proportioned head, 

 body, and feet. It is unpigmented save for the dark ring of 

 the iris, which is visible through the translucent eyelids. The 

 ears are folded forward and sealed. 



The new-born mouse left to itself may remain motionless 

 for as long as a minute before it gives its first tiny gasp, 

 followed by another and another. These gasps are con- 

 tinued at irregular intervals, often accompanied by violent 

 contractions of the body and especially the breathing 

 muscles. Within five minutes the mouseling is breathing 

 regularly and lying quietly except perhaps for an occasional 

 twitch of a leg or a sucking movement of the mouth. 



Nursing takes place as soon as the mother huddles over 



