522 MAMMALIA 



inferior vena cava, and thence in part passes into tlie right 

 auricle, but the greater part passes, in consequence of a special 

 arrangement of valves, directly into the left auricle through an 

 opening in the interauricular septum, called the foramen ovale. 

 The blood which reaches the right ventricle passes through a 

 vessel (ductus arteriosus Botalli), connecting the pulmonary 

 artery with the aorta, directly into the systemic circulation, 

 except a small portion which goes to the lungs. From 

 this condition of the circulation, it results that all the 

 arterial vessels contain mixed blood. The allantoic vein alone 

 contains pure arterial blood. 



As remains of the first stage of the circulation before the de- 

 velopment of the placenta, the omphalomeseraic (vitelline) 

 vessels — an artery and a vein — which belong to the umbilical 

 vesicle (yolk-sac), still persist. 



The duration of gestation depends on the size of the body and 

 the stage of development at which the young are born. It is 

 longest in the large terrestrial and the colossal aquatic animals 

 (Ungulata, Cetacea), which live under favourable conditions of 

 nourishment. The young of these animals are so far advanced 

 in their bodily development at birth, that they are able to follow 

 the mother (to a certain extent like praecoces). The period of 

 gestation is relatively shorter in the Carnivora, the young of 

 which are born naked and with closed eyes and, like altrices, are 

 for a long time completely helpless, and need the care and pro- 

 tection of the mother. It is, however, shortest in the aplacental 

 monotremes and marsupials. In these animals the young, 

 which are born at a very early stage (in the kangaroo they are 

 no larger than a nut), pass into a pouch formed by cutaneous 

 folds in the inguinal region, and here adhere firmly to the nipples 

 of the mammary glands. In this pouch, as in a second uterus, 

 they are nourished by the secretion of the mammary glands, 

 which assume at this early stage the nutrient function of the 

 absent placenta. The number of the young, which are born, 

 also varies very greatly in the different genera. The large 

 Mammalia, of which the period of gestation is longer than six 

 months, as a rule bear only one, more rarely two young ; but in 

 the smaller Mammalia and some domestic animals (pig) the 

 number is considerably larger, so that twelve to sixteen, or even 

 twenty young may be bom at one time. The number of teats on 



