THE INTEGUMENT 675 



embryo of many placental animals there is an entire ridge along which 

 the mammary glands are to appear. In a short time there are suppres- 

 sions at regular intervals which leave protruding nipples. These nipples 

 in turn become reduced and eventually become actual depressions. 



The varying position of the nipples in different groups of animals 

 is due to the retention of some of the nipples in a particular region and 

 the suppression of the remaining ones along this lateral ridge, which, 

 as mentioned, extends from axilla to groin. 



It is of interest that the aquatic Sirenia (Fig. 390) have pectoral 

 mammary glands. They bear but a single young at a time and. nurse 

 their offspring by standing erect in the water while clasping the young 

 in their flippers. It is supposed that many of the mermaid stories had 

 their origin from an observation of this animal nursing its young. 



It is by no means uncommon to find animals (including man) having 

 a peculiar arrangement of nipples on their bodies. Supernumerary nip- 

 ples are termed hyperthelism and supernumerary mammae are termed 

 hypermastism. These supernumerary developments sometimes occur on 

 the thigh and other parts of the body. They are considered displace- 

 ments and not reversions if they occur in out-of-the-ordinary regions, 

 and reversions if they occur in regions where they normally develop em- 

 bryologically. 



While rudimentary nipples occur in the male of placental mammals, 

 and may even prove to be functional in some instances, monotromes and 

 marsupial males do not develop them at all. 



SCALES 



As the dog-fish has what is called the indifferent type of an exoskel- 

 eton, it is this animal which forms the classic example for a preliminary 

 study. Here we find imbricated rows of pointed scales (which merely 

 means that one row of scales covers the intervals of the next row). 

 (Fig. 401.) The scales of other fishes, as well as of reptiles, and even 

 the feather papillae of birds, and the hair of mammals, are all arranged 

 in a similar manner. 



The scales of the dog-fish are said to be placoid (Fig. 401), which 

 means that each has an approximately flat base from which a sharp- 

 pointed cusp arises. This cusp is inclined in the direction of the free 

 edge of the scale. When the scale is in place, the inclination is toward 

 the posterior portion. 



The scale itself consists of a core of dentine which is overlaid with 

 enamel. In fact, the cusp is almost all enamel. The papilla from which 

 nourishment comes to the scale lies beneath. In the embryo the scale 

 forms between epidermis and corium, the dentine arising from the corium 

 and the enamel from the epidermis. 



In the selachians there are several rows of pointed teeth arranged 

 quite like the scales on the surface. These develop from the same layers 



