22 ZOOLOGY 



grasps by the basic hand may be the acid portion of still another 

 amiiio-acid. We see that in this way ammo-acids can be 

 linked together in long chains, and when proteid is broken up 

 by boiling it with acid, what happens is that the links in 

 these chains become separated from one another. Now an 

 acid can be di- or even tri- basic that is to say, that it may 

 have two or even three acid hands instead of one, and so we 

 see that there are possibilities of infinite complication in the 

 structure of these compound amino-acids. But we know that 

 proteid contains sulphur as well as the four elements named 

 above; there must be therefore a sulphur-containing group 

 attached somewhere in the amino-acid chain, and chromatin 

 is known to be a compound of proteid with nucleic acid, which 

 itself is a compound of phosphoric acid. We need not pursue 

 the subject further ; enough has been said to show that if the 

 living substance be not widely different in structure from dead 

 proteid, and if proteid be composed of enormously long amino- 

 acid chains, then, even if every different kind of animal and 

 plant has a different kind of living substance, there is enough 

 possibility of variation in the amino-acid chain to account for 

 them all. So far, this is by far the most plausible theory of 

 the composition of living substance which has yet been put 

 forward; but the working out of this theory in detail will 

 consume many decades of experiments, and it is on this work 

 in detail that its final verification depends. The best and final 

 proof of its truth would be the building up of proteid out 

 of simple substances in the laboratory. Some steps in this 

 direction have been taken, but at every step it is as if one were 

 following a road which continually forked and there were no sign- 

 post to guide one as to which fork one should follow. 



CHAPTER IV 



THE VARIOUS KINDS OF CELLS I THE TISSUES. 



WE have seen that the living substance in the bodies of the 

 larger animals is divided into cells. Now these cells are 

 never all alike, but differ from one another in size, shape, 



