INTEGUMENT 29 



skin remains almost on a level with the rest of the integument 

 {G). In the latter case the teats may be described as U'jic or 

 .secondary (Marsupials, Eodents, Lemurs, Monkeys, and Man), and 

 in the former as 2^seudo- or primary teats (Carnivora, Pigs, Horses, 

 and Ruminants). The latter condition is already indicated in 

 certain Marsupials (e.g. Phalangista vulpina). 



The number of teats varies greatly : there may be as few as 

 one pair, or as many as eleven pairs (Ceutetes). They are often 

 situated in two nearly parallel rows along the ventral side of the 

 thorax and abdomen which slightly converge towards the inguinal 

 region : in other cases they may be restricted either to the inguinal 

 (Ungulates and Cetaceans) or to the thoracic region (Sloths, 

 Elephants, Sirenia, many Lemurs, Cheiroptera, and Primates) : 

 while in others, they may be axillary or abdominal, or they may 

 ■occur in various combinations of all these regions. 



In the male, the mammary apparatus becomes aborted, though usually at 

 birth and puberty milk is produced in the human subject. Male goats and 

 ■castrated sheep have also been known to give milk, and the same is probably 

 true of male Bats. The occasional existence in men of supernumerary teats, 

 and in women of supernumerary mammje and teats {pobimastiam and ])"^]l- 

 thelism) is very remarkable. They are usually situated in the thoracic region, 

 and must be considered as atavistic to a primitive form which possessed 

 numerous teats and which produced a number of young at a time. Such a 

 transition from polymastism to bimastism may be seen plainly at the present 

 day in the Lemurs : in them the inguinal and abdominal teats are seen in 

 various degrees of retrogressive metamorphosis, while a single pair of thoracic 

 teats remain well developed. This accords with the fact that most Lemurs 

 bear only a pair of young ones at a time, which they carry with them at the 

 breast. Moreover, in various Mammals a greater number of teats are present 

 in the embryo than in the adult. 



The mammary glands, which are at first solid, become secondarily 

 hollowed out and further differentiated. The whole intermediate 

 tissue during lactation is filled with white blood-corpuscles 

 (leucocytes) ; and possibly the well-known structural elements of 

 milk, known as colostrums and milk-spheres, owe their origin to 

 these corpuscles, which have passed through the walls of the acini. 



