22 AN INTRODUCTION TO ZOOLOGY 



and flattened with a bulged central portion ; these are a pale 

 yellowish-red colour, and are called the red corpuscles. The others 

 are less numerous, smaller and colourless. They also differ from the 

 red in that they are not of a definite shape, and if watched carefully 

 will be found to change their shape slightly. They are termed the 

 white corpuscles or leucocytes. If a drop of weak acetic acid and 

 some dye, such as methyl-blue or methyl-green, be added to the 

 slide,* an alteration will be seen in both sorts of corpuscles. They 

 both become faintly tinged with the dye, but within them will 

 appear a definite circumscribed part that becomes much more 

 deeply stained. This is the nucleus, and all the corpuscles will 

 contain one such body, while certain of the leucocytes may contain 

 more than one. 



As we have learned already, living matter is composed of a 

 substance called protoplasm, and now we have seen that in the blood 

 the protoplasm is in the form of small pieces, the corpuscles. VHthin 

 the corpuscle itself, however, we find that at least two parts are to 

 be distinguished ; a central more dense portion that stains deeply 

 with certain dyes, and, secondly, a surrounding zone, less dense, 

 that stains more lightly. Such a small living mass we designate a 

 cell, and it may be defined as a mass of protoplasm, usually minute, 

 consisting of a central dense deeply-staining nucleus surrounded by a 

 clearer, more lightly staining material, usually termed the cytoplasm. 

 The corpuscles are very simple types of cells, and do not exhibit the 

 modifications that we find in the cells which with their products 

 constitute the entire body. 



This preliminary investigation of the frog has served to 

 enlarge our conception of a complex animal. We have seen that it 

 is capable of swimming, jumping, and performing a number of 

 voluntary actions that represent a certain amount of work, and need 

 the expenditure of a corresponding amount of energy. Moreover, 

 a very casual examination of the living frog is sufficient to show 

 that the movements of the various parts of the body are not 

 spasmodic, but are co-ordinated in such a way as to produce a 

 definite result. The frog, like all other animals, increases in size, 

 and by means of eggs is able to produce a number of animals, tad- 

 poles, which, after undergoing a series of changes, become similar 

 to their parents. The energy for these various activities is obtained 

 in a potential form from the food, which is first digested and then 

 assimilated, or made part of the body of the animal. The energy 

 thus stored is .released by slow combustion or oxidation, and the 



* This process will be hastened if the acetic acid and dye be placed on one 

 side of the cover-slip and the excess of fluid drawn off from the oppos ite side 

 by means of a piece of blotting-paper, thus causing the two fluids to be drawn 

 under the cover-slip much more rapidly. 



