CLASS MAMMALIA. MAMMALS 



475 



Development 



The egg is fertilized and undergoes seg- 

 mentation in the oviduct; it then passes into 

 the uterus, where it receives nourishment 

 and oxygen, and disposes of wastes by way 

 of the mother's blood circulation through a 

 structure called the placenta. This is formed 

 from the fetal membranes and united with 

 the mucous membrane of the uterine wall. 

 The interval between fertilization and birth, 

 known as the period of gestation, is 60 days. 



STRUCTURES OF 

 OTHER MAMMALS 



Hair 



The hairs which distinguish mammals 

 from all other animals (Fig. 345), unlike 

 feathers which are modifications of horny 

 scales, are new structural elements of the 

 skin. However, in function the hair and 

 feathers are similar in that both are insulat- 

 ing devices. When hairs are shed, new hairs 



Medullq 



Pigment 

 granules 



Cuticular scale 



Diagram of a 

 sectioned humon heir 



Human 



Mink 



Sheep 



Cat 



Cuticular scales 



Figure 345. Structure of various mammalian hairs. The hair is a complex structure, covered 

 with a sheath of scales, which superficially suggest shingles on a roof. The fact that many mam- 

 mals can be identified from the external cuticular scales makes a knowledge of them useful in 

 both crime detection and the fur industry. 



usually arise to take their place. Secretions 

 from the sebaceous glands keep the hairs 

 glossy. 



The two main types are (1) guard hairs 

 which are long and strong, and (2) woolly 

 hairs which are shorter and constitute the 

 underfur. In some animals the woolly hairs 

 have a rough surface, as in the sheep, which 

 causes them to cohere and gives them their 

 felting quality. Certain of the stronger hairs 

 may be moved by muscle fibers. The muscles 

 of the skin are responsible for the erection 

 of spines or the bristling of the other hairs. 



Scales 



Scales are present on the bodies of a few 

 mammals, notably on the pangolin (Fig. 



354), and on the tails of certain rodents such 

 as the beavers, rats, and mice. 



Claws, nails, hoofs, etc. 



These are all modifications of the horny 

 covering on the dorsal surface of the distal 

 ends of the digits (fingers and toes). The 

 chief forms are shown in Fig. 346. When on 

 the ground the foot rests partially or en- 

 tirely upon the digital pads. Dermal papillae 

 occur on the digital pads, often forming 

 concentric ridges such as those that produce 

 the fingerprints of man. The sole is softer 

 than the nail plate. 



Other epidermal horny thickenings are 

 the hornsheaths of the ox and other rumi- 

 nants, the nasal horns of the rhinoceros, and 



