76 COMPARATIVE ANATOMY 



ingest many other substances such as bacteria, this pheno- 

 menon being known as phagocytosis. The fat granules 

 may be subsequently extruded from the corpuscle, or they 

 may be used up and converted into its substance, and 

 therefore the leucocyte is assimilative and metabolic; for, 

 although we cannot measure the expenditure of energy nor 

 trace the formation and rejection of waste material in so small 

 a body, we are certain that the exhibition of energy in the 

 form of pseudopodial movement must involve a chemical 

 change ; a decomposition of the chemical substances of which 

 the corpuscle is composed, and the production of waste matters 

 as a result of the decomposition. And the leucocytes are 

 reproductive, for they may be observed to divide into two, 

 each portion containing a part of the nucleus, and the daughter 

 leucocytes thus formed carry on the existence of the parent 

 form which gave origin to them, and lead independent exist- 

 ences as individual leucocytes. In short, the leucocyte mani- 

 fests all the activities characteristic of a living being, and 

 withal it is nothing more than a speck of. that peculiar life- 

 substance which we call protoplasm, showing scarcely any 

 structure, beyond that it is divisible into nucleus and cell-body 

 for the leucocyte is a fairly typical example of what is called a 

 cell ; and, before we consider the different kinds of leucocytes 

 which occur in the blood and lymph of a frog, we may pause 

 for a moment to consider what a cell is. 



" A cell is a corpuscle of protoplasm which contains within 

 it a specialised component particle, the nucleus." Such is the 

 definition of a cell given by one of the most notable living 

 authorities on the subject, and for our present purposes the 

 definition may be accepted. But it must be remembered that 

 it is well-nigh impossible to frame satisfactory definitions 

 concerning living things. " A cell is a corpuscle of proto- 

 plasm containing a nucleus." But we have just seen that a 

 leucocyte, a single corpuscle of protoplasm, may contain two or 

 more nuclei. Is it still a cell ? or, does the increase of the 

 number of nuclei remove it from our definition ? In the case 

 of the leucocyte, certainly not. The double, treble, or 

 quadruple nucleus is clearly brought about by division of a 

 single nucleus, and the parts are smaller than the whole from 

 which they were formed. And it appears that when a multi- 

 nuclear leucocyte reproduces itself by division each nuclear 



