62 THE BODY AT WORK 



be seen as colourless discs. Haemoglobin constitutes 40 per 

 cent, of the weight of a moist corpuscle, or 95 per cent, of its 

 weight after it has been dried. This is an enormous charge 

 for a corpuscle to carry, and the question of how it carries it 

 has been much discussed. It is not in a crystalline state. 

 A corpuscle examined by polarized light is not doubly refrac- 

 tive. Microscopists know that if there were any crystals in 

 the corpuscle it would appear bright on a dark ground when 

 the Nicholl prisms are crossed. It cannot be in solution, since 

 the water which the corpuscle contains would not suffice to 

 dissolve it. It must be combined with some constituent of 

 the corpuscle. But whether it is uniformly distributed 

 throughout the disc, or in a semifluid form enclosed in spaces 

 in a sponge-work ; or whether the corpuscle is a hollow 

 vesicle enclosing fluid haemoglobin a view which was long ago 

 maintained, and has recently been revived are questions 

 which still await further evidence. 



Red blood-corpuscles, properly so called, are found only in 

 vertebrate animals, although invertebrate animals, from worms 

 upwards, possess genuine blood, and in some of them it contains 

 haemoglobin, or a similar pigment in the form of globules. 

 These might be likened to the non-nucleated corpuscles of 

 mammals, but it must be remembered that the non-nucleated 

 cells of mammals have been evolved from the nucleated blood- 

 corpuscles of birds, reptiles, amphibians, and fishes. Below 

 fishes red blood-cells are not found. Haemoglobin is usually 

 dissolved in the blood of invertebrate animals. It is impossible 

 to trace any relationship between the coloured globules of 

 invertebrates and the blood-cells of fishes. The coloured 

 globules must be regarded as deposits or accretions of haemo- 

 globin held together by a proteid substance. 



The nucleated red corpuscles of submammalian vertebrates 

 multiply by cell division while circulating in the blood-stream. 

 A good subject in which to look for dividing corpuscles is the 

 blood of a newt in spring-time, when rapidly increasing activity 

 calls for an additional supply. There is nothing to distinguish 

 the method of division of a nucleated blood-corpuscle from that 

 of any other cell. 



The life-story of the red blood-corpuscles of mammals is one 

 of the most fascinating that the histologist has to tell. He 



