CHAPTER III 

 THE HISTOLOGY OF THE FROG 



WE must now consider shortly the ultimate elements of which 

 the complex mechanisms of the frog are built up, and we shall 

 begin best by taking the simple case of the blood. 



The blood differs slightly in colour according as it is taken 

 from an artery or a vein. In the former case it is bright 

 scarlet in colour, in the latter dark and almost purple in hue, 

 but in both cases the microscopical characters are the same. 

 The blood is an opaque viscid fluid, slightly alkaline in re- 

 action, and consists of a fluid matrix, the plasma, in which 

 solid particles of minute size, the corpuscles, are contained. 



The plasma is a transparent fluid, of a pale yellow colour, 

 containing albumens and some salts in solution. The cor- 

 puscles are of two kinds, red and white, and the white 

 corpuscles may further be divided into three kinds. The 

 red corpuscles, or hsematids, are far more numerous than the 

 white. Each is a flattened oval disc, measuring some 0*023 

 mm. in its widest and some 0*014 mm - in its shortest 

 diameter ; it is pale red or yellow in colour, and presents a 

 central bulging due to the presence of a specialised central 

 portion, the nucleus. If a film of blood is treated with certain 

 dyes, such as carmine, hsematoxylin (a preparation of log- 

 wood), magenta, etc., the flattened part or body of the 

 corpuscle is but little affected by the stain, but the nucleus 

 absorbs it readily, and may be seen to contain a network of a 

 substance which stains more deeply than the rest. This intra- 

 nuclear network, because of its strong affinity for certain 

 colouring matters, is called chromatin. The body of the 

 corpuscle may also be observed to be made up of a network 

 of a denser substance containing a more fluid substance in its 

 meshes. The red colour of the body of the corpuscle is due 

 to the presence of an iron-holding compound named haemo- 

 globin, which can be separated out by shaking up blood with 



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