CHAPTER XXXV 



BLOOD RELATIONSHIPS AMONG THE CHORDATES 



The various species of animals differ not only in their structure, 

 their method of development and their habits, but also in the 

 chemical composition of their tissues. The most useful tissue 

 to take in this connexion is the blood. Now, chemical methods 

 are not sufficiently refined to detect the difference between the 

 bloods of two animals and to estimate the degree of similarity 

 which they show. It is possible, however, to have recourse to 

 biological methods by making use of the property which animals 

 possess of developing immunity. If horse 's blood , for example , 

 is injected into the vascular system of a rabbit, the rabbit will 

 after a time produce a substance in its blood which reacts to 

 horse's blood, and precipitates it. This is the same principle 

 as that used for preparing antitoxins for certain diseases. As 

 to how the antitoxin or antiserum is produced, little is known, 

 but it suffices for present purposes to realise that in the hypo- 

 thetical case just described, rabbit's blood immunised against 

 horse's blood will always precipitate horse's blood, to the 

 extent of ioo per cent. This means that anti-horse serum, 

 as it may be called, is specific against horse, and it is a matter of 

 no importance what kind of animal has been used to produce 

 the antiserum. But the specificity against horse is not quite 

 complete. Anti-horse serum, as it may be called, will produce 

 no effect whatever if mixed with, say, blood of a bird ; but it 

 will produce a slight precipitation with blood of pig, and still 

 more with blood of ass. This means that the blood of horse is 

 more similar to that of ass than to that of pig, as regards its 

 chemical composition, and this is just what would be expected 



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