MAMMALIA— PHYSIOLOGY 423 



Most jTvam-mals except the ungulates have a perfected digitigrade 

 gait, having developed special sole-pads for the absorption of the 

 shock. The elephant^ however, is said to be rectigrade, its weight 

 resting on heavy pads, the foot not moving independently. In the 

 camel^ the feet are provided with two broad cushion-like pads, while 

 the hoofs are reduced to nails. 



The only tvuQ flying mammals are the bats, but other forms, such 

 as certain squirrels, are able to plane through the air. Extensions 

 of the skin on the inside of the fore and hind limbs are drawn taut 

 when the limbs are extended, and furnish a flat surface. 



Some forms like the mole and the gopher are able to burrow in 

 the ground. In these forms, particularly in the mole, one finds 

 heavily developed shoulder girdles. Depending on the degree of 

 subterranean life, one finds degeneration of the eyes. 



Certain aquatic mammals such as the whale have undergone 

 degeneration of the hind limbs, with a great development of the 

 pectoral appendages for swimming. In marine mammals, we find 

 that the caudal fin is horizontal instead of vertical and that it has 

 two symmetrical halves. 



C&nfro/ bodies CcenfrosomK) 



Oc^g, 6o<:/,B^l,-^y ^ Vc^^Z/co/ layer 



PJosmosame or true nucleo/us "^~-T-^ *'j-ii"'A^ ^ *^'r"1 



Chromatin "T Tfeul^itSiiSr; -■.*:- -14 Plasfids 



Linin 



'■^Truc trva/f or membrono 



Korvosome or chromatin- nucleolus It-': < ^ , ^ 'a « ^ '•ri ^ . . 



Vacuole ff/tjid <^obule) 



- Passive metaplosmic or 

 paroplaatic bodies 



Fig. 236. Diagram of a generalized cell. (After Wilson, The Cell in Development and 

 Heredity. Courtesy of The Macmillan Co.) 



Histology. Cells and Tissues. — All living bodies are composed 

 of organic constituents called cells. In the Metazoa we find one 

 fundamental characteristic, that of division of labor. Certain cells, 

 grouped together for the purpose of reproduction, are called germ 

 cells, while others which carry on all processes except reproduction 

 are called somatic cells. The argument between the Lamarckian 

 and the Weismannian schools is based upon the question of possible 

 influence of environment upon the germ cells. (See page 514.) 

 (Figure 236.) 



