120 STRUCTURE OF THE VERTEBRATES 



glands of internal secretion. Milk glands are normally found on 

 the ventral surface of the body, varying widely as to number 

 and position. 



The monotremes (page 76) have the glands opening into 

 slight depressions on the posterior abdominal wall. The young 

 lap the milk, much as a kitten licks milk from a saucer. The 

 marsupials (page 77) have developed teats to which the young 

 become attached after birth. In the opossum there is a single 

 gland duct leading into each teat. 



The placentals usually have several ducts leading into a teat. 

 In the more primitive type, as in the primates, the ducts empty 

 directly to the outside at the end of the teat. In others, as the 

 cow, there is an inpocketing of ectoderm at tlie nipple, forming 

 a slight reservoir in the teat with the milk ducts opening proxi- 

 mally into it. The increased number of ducts in the placental 

 gland furnishes a more abundant supply of milk to the more 

 highly developed young. 



The primitive placentals had a double row of mammae along 

 the abdomen, but in the more specialized orders tliey are more 

 limited in number and located either pectorally or inguinally. 

 In the embryos of these animals there are usually several pair 

 which do not develop; but accessory, functionless, nipples are 

 frequently found in both sexes of the human and other placentals. 



6. Horns. Horns and antlers, which are found in the artio- 

 dactyls (page 87), are distinct in development and structure. 

 Horns have a permanent bony core with an epidermal covering, 

 and the entire structure is permanent throughout life. The Long 



^Frontal bone 



Fig. 57. Growth and Structure of Horns. Horns are permanent, with a 

 bony core and an ectodermal cover. 



