32 COMPARATIVE ZOOLOGY. 



the cell-contents. 18 The nucleus is generally attached to 

 the inside of the membrane, and is the centre of activity. 



Cells vary greatly in size, but are generally invisible to 

 the naked eye, ranging from --J-g- to T-JTUTTO of an inch in 

 diameter. About 4000 of the smallest would be necessary 

 to cover the dot of this letter i. The natural form of iso- 

 lated cells is spherical; but when they crowd each other, 

 as seen in bone, cartilage, and muscle, their outlines be- 

 come angular, either hexagonal or irregular. 



Within the narrow boundary of a simple sphere, the 

 cell-membrane, are exhibited all the essential phenomena 

 of life growth, development, and reproduction. The 

 physiology of these minute organisms is of peculiar inter- 

 est, since all animal structure is but the multiplication of 

 the cell as a unit, and the whole life of an animal is that 

 of the cells which compose it: in them and by them all 

 its vital processes are carried on. 14 



The structure of a cell can be seen in blood-corpuscles, 

 by diluting with a weak (J- per cent.) solution of salt a 

 drop of blood from a Frog, and placing it under the mi- 

 croscope. (See Fig. 63.) 



2. Tissues. There are organisms of the lowest grade 

 (as Gregarina) which consist of a single cell, living for and 

 by itself. In this case, the animal and cell are identical: 

 the Gregarina has as much individuality as the Elephant. 

 But all animals, save these unicellular beings, are mainly 

 aggregations of cells : for the various parts of a body are 

 not only separable by the knife into bones, muscles, nerves, 

 etc., but these are susceptible of a finer analysis by the 

 microscope, which shows that they arise from the devel- 

 opment and union of cells. These cellular fabrics, called 

 tissues, differ from one another both chemically and struct- 

 urally, but agree in being permeable to liquids a prop- 

 erty which secures the flexibility of the organs so essential 

 to animal life. Every part of the human body, for exam- 



