450 APPLIED BIOLOGY 



Such a cavity adapted to holding embryos is called a uterus 

 or womb. In some low mammals, like kangaroos and opos- 

 sums, there is a right and a left uterus formed by expansion 

 of part of each oviduct. These lie in the same position as the 

 oviducts of frogs. In higher mammals the right and left uteri 

 grow together during embryonic life, and so there is a single 

 uterus with a tube (Fallopian) extending to each ovary (right 

 and left). Egg-cells formed and discharged by either ovary 

 pass through the tube into the uterus and there develop 

 into embryos. 



In all mammals the egg-cells discharged are fertilized near 

 the ovaries in the Fallopian tubes by sperm-cells which have 

 arrived there by swimming through the secretions on the liv- 

 ing membranes of the uterus and tubes. The fertilized egg 

 begins to divide at once and may have undergone con- 

 siderable development when, after a few days, it slips from 

 the tube into the uterus. 



The number of egg-cells fertilized at one time varies in 

 different species. It is well known that many domesticated 

 animals (e.g., sheep, cow, horse) usually have one offspring 

 at a time; but some occasionally produce two (twins) or 

 even three (triplets). Others commonly produce many 

 young at a time (e.g., dog, cat, pig, rabbit, mice). The num- 

 ber of young produced indicates the number of egg-cells 

 which were matured and fertilized. 



The period of development in the uterus from fertilization 

 to birth of the young is commonly known as gestation or 

 pregnancy; and the length of time is highly variable. It is 

 approximately 21 days in guinea-pig, 30 days in rabbit and 

 squirrel, 55 days in cat, 62 days in dog, 3 months in lion, 

 4 months in pig, 5 months in sheep and goat, 6 months in 

 bear, 9 months in cow, over 9 months (280 days) in human 

 species, 10 months in whale, 11 months in horse, 14 months 

 in giraffe, and 22 months in elephant. These are simply 

 illustrations selected from familiar mammals. 



