462 ZOOLOGY SECT. 



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. 



The segmentation is of the holoblastic type. An amnion and 

 an allantois are developed much as in the case of the Bird (p. 428). 

 But the later history of these foetal membranes is widely different 

 in the Rabbit, owing to the modifications which they undergo, in 

 order to take part of the formation of the placenta the structure 

 by whose instrumentality the foetus receives its nourishment from 

 the walls of the uterus. The placenta is formed from the serous 

 membrane or chorion the outer layer of the amniotic folds in a 

 limited disc-shaped area, in which the distal portion of the allantois 

 coalesces with it (Fig. 1110). The membrane thus formed develops 

 vascular processes the chorionic villi which are received into de- 

 pressions (the uterine crypts] in the mucous membrane of the uterus. 

 The completed placenta with its villi is supplied with blood by the 

 allantoic vessels. The placenta of the Rabbit is of the type termed 

 deciduate, the villi of the placenta being intimately united with 

 the uterine mucous membrane, and a part of the latter coming 

 away with it at birth in form of a decidua, or after-birth. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION OF RECENT 



MAMMALIA. 1 



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 characteristic of the spinal column of Mammals are the discs 

 of fibre-cartilage termed intervertebral discs, which intervene 

 between successive centra. 



The skull has two condyles for connection with the atlas, instead 

 of the single condyle of the Sauropsida ; and the lower jaw, which 

 consists of only a single bone on each side, articulates with the 

 skull in the squamosal region without the intermediation of the 

 separate quadrate element always present in that position in Birds 

 and Reptiles. 



Each of the long bones of the limbs is composed in the young 

 condition of a central part or shaft and terminal epiphyses, the 

 latter only becoming completely united with the shaft at an ad- 

 vanced stage. 



In the pectoral arch the coracoid of the Birds and Reptiles is 

 usually represented only by a vestige or vestiges, which unite with 

 the scapula in the adult. The ankle-joint is always situated between 

 the tibia and the tarsus. 



1 Extinct groups are referred to in dealing with the distribution. 



