28 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



planation. Most physiologists unquestionably believe that further experi- 

 mental work will bring this phenomenon out of its obscurity and show that it 

 is explicable in terms of known physical and chemical forces acting through 

 the peculiar substance of the absorptive cell. 



The facts of heredity and consciousness offer difficulties of a much graver 

 character. The function of reproduction is two-sided. In the first place 

 there is an active multiplication of cells, beginning with the segmentation of 

 the ovum into two blastomeres and continuing in the larger animals to the 

 formation of an innumerable multitude of cellular units. In the second place 

 there is present in the ovum a form-building power of such a character that 

 the great complex of cells arising from it form not a heterogeneous mass, but a 

 definite organism of the same structure, organ for organ and tissue for tissue, 

 as the parent form. The ovum of a starfish develops into a starfish, the 

 ovum of a dog into a dog, and the ovum of man into a human being. 

 Herein lies the great problem of heredity. The mere multiplication of cells 

 by direct or indirect division is not beyond the range of a conceivable me- 

 chanical explanation. Given the properties of assimilation and contractility it 

 is possible that the act of cell-division may be traced to purely physical and 

 chemical causes, and already cytological work is opening the way to credible 

 hypotheses of this character. But the phenomena of heredity, on the other 

 hand, are too complex and mysterious to justify any immediate expectation 

 that they can be explained in terms of the known properties of matter. The 

 crude theories of earlier times have not stood the test of investigation by 

 modern methods, the microscopic anatomy of both ovum and sperm showing 

 that they are to all appearances simple cells which exhibit no visible signs of 

 the wonderful potentialities contained within them. Histological and experi- 

 mental investigation has, however, cleared away some of the difficulties for- 

 merly surrounding the subject, for it has shown with a high degree of prob- 

 ability that the power of hereditary transmission resides in a particular sub- 

 stance in the nucleus, namely in the so-called chromatin material which forms 

 the chromosomes. The fascinating observations which have led to this con- 

 clusion promise to open up a new field of experimentation and speculation. 

 It seems to be possible to study heredity by accepted scientific methods, and 

 we may therefore hope that in time more light will be thrown upon the 

 conditions of its existence and possibly upon the nature of its activity. 



In the facts of consciousness, lastly, we are confronted with a problem 

 seemingly more difficult than heredity. In ourselves we recognize different 

 states of consciousness following upon the physiological activity of certain 

 parts of the central nervous system. We know, or think we know, that these 

 so-called psychical states are correlated with changes in the protoplasmic 

 material of the cortical cells of the cerebral hemispheres. When these cells 

 are stimulated, psychical states result; when they are injured or removed, 

 psychical activity is depressed or destroyed altogether according to the extent 

 of the injury. From the physiological standpoint it would seem to be as 

 justifiable to assert that consciousness is a property of the cortical nerve-cells 



