522 MANUAL OF ZOOLOGY sect. 



as the intra-uterine embryo is termed. The young animal 

 escapes from the uterus in a condition in which all the parts 

 have become fully formed, except that the eyelids are still 

 closed, and the hairy covering is not yet completed. As 

 many as eight or ten young are produced at a birth, and 

 the period of gestation, i.e., the time elapsing between the 

 fertilisation of the ovum and the birth of the young animals 

 is thirty days. Fresh broods may be born once a month 

 throughout a considerable part of the year, and, as the 

 young rabbit may begin breeding at the age of three 

 months, the rate of increase is very rapid. 



During intra-uterine life the young rabbit is nourished by 

 an organ called the placenta, formed by an intimate union 

 between certain structures, the fcetal membranes, derived 

 from the embryo, and a specially modified part of the wall 

 of the uterus. By means of the placenta a close connection 

 is established between the blood-system of the foetus and 

 that of the parent, and nourishment is thus received by the 

 former from the latter. 



After birth the young rabbits are nourished for a time 

 wholly by the milk or secretion of the mammary glands of 

 the mother. 



The following are the principal general features which 

 characterise the Mammalia as a class : — 



The Mammalia are air-breathing vertebrates, with warm 

 blood, and with an epidermal covering in the form of hairs. 

 The bodies of the vertebrae are in nearly all mammals 

 ossified each from three independent centres, one of which 

 develops into the centrum proper, while the others give 

 rise to thin discs of bone, the epiphyses. Also charac- 

 teristic of the spinal column of mammals are the discs of 

 fibro-cartilage termed inter-vertebral discs, which intervene 

 between successive centra. 



