22 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



in certain properties with a concomitant suppression of other properties, the 

 outcome of which is that muscular tissue exhibits pre-eminently the power of 

 contractility, the nerve tissues are characterized by a highly developed power 

 of conductivity, and so on. While in the simple unicellular forms of animal 

 life the fundamental properties are all somewhat equally exhibited within the 

 compass of a single unit or cell, in the higher animals we have to deal with 

 a vast community of cells segregated into tissues each of which possesses some 

 distinctive property. This specialization of function is known technically as 

 the physiological division of labor. The beginning of this process may be 

 recognized in the cell itself. The typical cell is already an organism of some 

 complexity as compared with a simple mass of undifferentiated protoplasm. 

 The protoplasm of the nucleus, particularly of that material in the nucleus 

 which is designated as chromatin, is differentiated, both histologically and 

 physiologically, from the protoplasm of the rest of the cell, the so-called cyto- 

 plasm. The chromatin material in the resting cell is arranged usually in a 

 network, but during the act of division (karyokinesis) it is segmented into a 

 number of rods or filaments known as chromosomes. In the ovum there are 

 good reasons for believing that the power of transmitting hereditary charac- 

 teristics has been especially acquired by these chromosomes. The nucleus, 

 moreover, controls in some way the metabolism of the entire cell, for it has 

 been shown, in some cells at least, that a non-nucleated piece of the cytoplasm 

 is not only deprived of the power of reproduction, but has also such limited 

 powers of nutrition that it quickly undergoes disintegration. On the other 

 hand contractility and conductivity, and some of the functions connected with 

 nutrition, such as digestion and excretion, seem often to be specialized in the 

 cytoplasm. As a further example of differentiation in the cell itself the ex- 

 istence of the centrosome may be referred to. The centrosome is a body of 

 very minute size which has been discovered in numerous kinds of cells. It 

 is considered by many observers to be a permanent structure of the cell, lying 

 either in the cytoplasm, or possibly in some cases within the substance of the 

 nucleus. When present it seems to have some special function in connection 

 with the movements of the chromosomes during the act of cell-division. In 

 the many-celled animals the primitive properties of protoplasm become highly 

 developed, in consequence of this subdivision of function among the various 

 tissues, and in many ways the most complex animals are, from a physiological 

 standpoint, the simplest for purposes of study, since the properties of living 

 matter become separated and emphasized in them. to such an extent that they 

 are better fitted for accurate observation. 



We are at liberty to suppose that the various properties so clearly recognizable 

 in the differentiated tissues of higher animals are all actually or potentially 

 contained in the comparatively uudifferentiated protoplasm of the simplest uni- 

 cellular forms. That the lines of variation, or in other words the direction of 

 specialization in form and function, are not infinite, but on the contrary 

 comparatively limited, seems evident when we reflect that in spite of the 

 numerous branches of the phylogeuetic stem the properties as well as the 



