IXTEGUMEXT OF VEETEBRATA. 



421 



looks like an appendage of the gland. The sebaceous glands 

 undergo the most various modifications in form, size, and number, 

 as well as in the quality of their secretion. Both sets of glands 

 frequently secrete odorous matters of various kinds, which play an 

 important part in the economy of the animal. Glands of this 

 kind are developed in the most diverse regions of the surface of the 

 body in many orders of the Mammalia. 



§ 323. 



The most important modification of the integu- 

 mentary glands in all Mammals is the development of 

 milk-secreting glands, which enter into relation to the reproduc- 

 tive function. They are regularly, and, as a rule, symmetrically 

 arranged on the ventral surface of the body. Each " mammary, 

 gland " consists of a complex of separate glandular tubes, the ducts 

 of which are either quite separate or united together. 



In the Monotremata these organs differ but little from the other 

 kinds of integumentary glands. Each of the two organs here 

 present is made up of a 



group of tubes, which „ A 



pass separately through 

 the skin. The area on 

 which they open is merely 

 distinguished by the ab- 

 sence of hairs, and in 

 Ornithorhynchus is on 

 the same level as the 

 surrounding integument. 

 In Echidna it is placed 

 inapouch-like depression 

 (mammary pouch), which 

 appears to serve as a re- 

 ceptacle for the young. 



In the rest of the 

 Mammalia nipples are 

 present; these are special 

 arrangements which were 

 gradually developed by 

 the process of sucking, 

 and which afford the 

 young a more suitable 

 connection with the mam- 

 mary apparatus, while at 



Fig. 217. Diagram of the development of the 

 nipple ; vertical section. A Indifferent stage ; 

 glandular area flat. B Elevation of the glandular 

 area -with, the nipple. C Elevation of the peri- 

 phery of the glandular area into the pseudo- 

 nipple, a Periphery of the glandular area, b Glan- 

 dular area, tjl Glands. 



the same time they make 



each complex of milk-glands distinguishable externally. 



There are two very different conditions of the perfected 

 nipple. They are both preceded by a similarly indifferent stage 

 (Fig. 217, A), in which a nearly flat glandular area (b) has a number 



