COMPARATIVE PATHOLOGY. 749 



import of pigments, before the vexed subject of their 

 secondary import can be definitely dealt with. 



CoiMPARATivE Pathology. 



Within recent years pathologists have begun to study 

 diseased conditions comparatively — an obviously rational 

 method which promises to lead to very important results, 

 both practical and theoretical. For man has no monopoly 

 of disease, and some of the processes by which unhealthy 

 conditions are dealt with by the organism are more readily 

 studied in lower animals than in him. Of this we shall give 

 one illustration. In 1862, Hseckel observed that grains of 

 indigo injected into the mollusc Thetys were surrounded by 

 the amoeboid blood corpuscles. Other observers followed 

 the hint which this suggestive fact supplied, and Metchnikoff, 

 above all others, has shown the important role which these 

 amcEboid cells fill in waging war against intruding germs 

 and parasites, in surrounding irritant particles, in repairing 

 injuries, and the like. In fact, Metchnikoff has worked out 

 the evolution of the phagocyte, as he terms the amceboid cell 

 whose function it is to discharge the role above indicated. 

 It is this evolution, as stated in Metchnikoff s lectures on 

 the comparative pathology of inflammation {Trans., London, 

 1893), which we shall take in illustration of comparative 

 pathology. 



The simplest conditions are of course illustrated by the 

 Protozoa. These enjoy comparative immunity from the 

 injurious effects of wounds and from infectious disease. 

 For injuries are very rapidly repaired ; a fragment, if nucle- 

 ated, can usually regrow the whole; infecting organisms are 

 in most cases digested, and irritant particles are got rid off. 

 This is particularly true of the amceboid Protozoa, the 

 Rhizopods. Sometimes, moreover, the Bacteria or other 

 micro-organisms which produce disease are actually avoided, 

 for some of the Protozoa exhibit that sensitiveness (or 

 chemiotaxis) which distinguishes the wandering amoeboid 

 cells or phagocytes of higher animals. Thus, a Myxomycete 

 will creep towards a decoction of dead leaves and away from 

 a salt solution, and will " prefer " a nutritive fluid which is 

 not swarming with Bacteria to one that is. 



