The period of gestation is the time between the impregnation of the ovum and the birth of the young. In egg-laying animals it is the period of incubation. The length of this period is subject to considerable variation, determined by various causes not well understood. In general its length is in relation to the size of the animal. The following list, and remarks, represents only a few animals and the period of gestation of each (F. B. Mumford): —

Elephant..............    20 to 30 months

Giraffe...............    14 months

Buffalo..............    10 to 12 months

Ass................    12 months

Mare...............    11 to 12 months

Cow................    9 to 91/2 months (285 days)

Bear................    6 months

Sheep and goat............    5 months (21 weeks)

Sow................    4 months

Beaver...............    4 months

Lion.................    31/2 months

Dog, fox, or wolf............    2 months

Cat................    50 days

Rabbit...............    30 days

Squirrel and rat............    28 days

The period of incubation extends as follows for domestic fowls: —

Turkey...............    26 to 30 days

Guinea...............    25 to 26 days

Pea hen...............    28 to 30 days

Ducks...............    25 to 32 days

Geese................    27 to 33 days

Hens................    19 to 24 days (average 21)

Pigeons...............    16 to 20 days

Canary birds.............    13 to 14 days

Small breeds hatch earlier. Hamburgs hatch at the end of the twentieth day ; game bantams at the end of the nineteenth day. Duck eggs hatch earlier under hens than under ducks, probably because of the higher temperature of the hens' body.

Small breeds of animals require rather less time than larger breeds, although early maturity shortens the time. Cold weather retards the process of incubation especially. According to Youatt, all animals vary greatly without any known cause. The period of gestation in a horse has been known to vary from ten to over twelve months. Tessier reports 582 cases among mares, with a range of 287 to 419 days ; 1131 cows ranged from 240 to 321 days. Earl of Spencer reported 764 cows with a range of 220 to 313 days. L. F. Allen reports results for one year among a herd of 50 Shorthorns, Herefords, and Devons, as ranging from 268 to 294 days, or an average of 284 days. Tessier observed 912 ewes with a range of 146 to 161 days. Darwin found that Merinos run about 150 days, while Shropshires and Southdowns require only about 144 days. Swine vary from 109 to 123 days, but usually run 116 days.

In practice there are some causes which hasten birth. A sudden cold spell will hasten the birth of a litter of pigs. Nervous excitement will hasten birth, especially in cows. Parturition of a neighboring cow often hastens birth. It is a popular opinion that male offspring require a longer period of gestation. There is not sufficient evidence to warrant this, but in one case of observation on cattle, the average period for five years was males 288 days, females 283 days. Heredity may influence the period somewhat.

Number of young at birth (Harper)

Elephant.........................1

Giraffe...........................1

Buffalo..........................1

Ass.............................1

Mare...........................1

Cow...........................1

Bear...........................2

Sheep.........................1-2-3

Sow...........................2-14

Beaver ..........................4

Lion...........................2

Dog..........................3-8