THE ANIMAL CELL. 319 



tions, as after copious hemorrhages, such nucleated corpuscles may 

 occur in the circulating blood in large numbers. In the bone- 

 marrow, where they are apparently formed, they are always present. 

 As all manifestations of life are intimately associated with 

 chemical changes which bring about a transformation of potential 

 into kinetic energy, such changes must of necessity occur in every 

 living cell. These changes, moreover, must vary with the func- 

 tion which the cell is to perform, and will hence differ, to a 

 certain extent at least, with the different tissues of the complex 

 organism. In the monocellular organisms, where all the various 

 functions are performed by the single cell, all these varying changes 

 must hence be represented. But it is to be inferred that in accord- 

 ance with the greater simplicity in structure the chemical changes 

 also must be of a simpler character. We should hence expect that a 

 study of the chemical processes which take place in such low forms 

 of life would furnish us with a better insight into the metabolism 

 of the complex organism than could be attained from an investiga- 

 tion of the higher forms. Unfortunately, however, this is almost 

 an impossibility with the usual chemical and physical methods, for 

 in attempting such a study we are met with a most serious obstacle, 

 viz., our inability to maintain the life of the individual cell during 

 such an investigation. We would consequently have no proof that 

 those products which we could isolate from the dead cells were 

 present as such in the living organism. The technical difficulties, 

 moreover, which stand in the way of such a study are almost insur- 

 mountable. With microchemical methods, it is true, something 

 more definite might be accomplished, and although this branch of 

 investigation is still in its infancy, it has already furnished us with 

 a certain amount of. valuable information. The great advances 

 which have thus been made in our knowledge of the structure 

 of cells have largely been accomplished in this manner. Upon 

 the chemical processes themselves, however, which take place 

 in the cell, not much light has as yet been thrown in this 

 manner. We are consequently dependent for our knowledge of 

 the metabolic processes which take place in the living body upon 

 a study of the individual tissues as such, and the changes which 

 result in certain substances when introduced from without. With 

 some tissues this is more difficult than with others. The most 

 satisfactory results, on the whoje, regarding the chemical structure 

 of the individual cell, have thus far been obtained from an investi- 

 gation of those organs which are especially rich in cells, and in 

 which the cells can be more or less completely separated from the 

 underlying matrix and from other components which may be 

 present at the same time. This is especially true of the leucocytes 

 of the blood. As these bodies, moreover, are but little differentiated, 

 they may well serve as types of primitive cells. They are all 

 nucleated, and contain a varying amount of protoplasm, which in 

 some is capable of progressive movement, A limiting membrane, 



