THE INTEGUMENT AND THE EXOSKELETON 115 



7 his type of nipple, in reality a mammary pocket turned inside 

 0'it, is the most usual among the higher animals. In it the 

 glandular area forms the nipple itself, while the corium wall, 

 the rampart-like lip of the pocket, becomes the areola, a circular 



FIG. 31. Morphology of nipples. [After WEBER.] 



(a) Primary condition, as in Echidna. (b) Embryo calf, comparable with the 

 condition seen in (a). (c) Cow (adult) showing " false " nipple produced by the 

 prolongation of the cutis wall, (d) Halmaturus (a marsupial) previous to lactation, 

 (e) Didelphys (a marsupial) previous to lactation, (f) Didelphys during lactation, 

 showing " true " nipple, produced by the eversion of the mammary pocket. 



In all the figures the area glandularis, i. e., the surface of the mammary pocket, 

 is represented by a dotted line; the cutis wall by a full line. The branching lines 

 opening either at the bottom of the depression or at the summit of the elevation, are 

 the milk glands. 



area of modified skin surrounding the nipple. Quite another 

 type of nipple, though derived equally with the former from 

 the mammary pocket of the Echidna, is that occurring in 

 ruminants. In this the glandular area remains at the bottom 

 of the pocket, while the surrounding corium wall becomes ele- 

 vated more and more until a long pendulous nipple is formed 

 from that (Fig. 31, b and c). The mammary pocket is 

 here retained as a long lactiferous duct running through the 

 nipple. In many placental mammals the earliest embryonic 

 indication of the mammary glands consists of a lateral ridge, 



