47 2 THE ORIGIN OF VERTEBRATES 



indeed, have been said again and again to give origin to the free- 

 living blood- corpuscles of the adult. In other cases they are said to 

 disintegrate after their work is done. 



In the adult the free-living lymphocytes and hremocytes reproduce 

 themselves from already existing free-living cells, but as we pass back 

 to the embryo there comes a time, comparatively late in the history 

 of the embryo, when such free-living cells are not found in the fluids 

 of the body, and they are said to arise from the proliferation and 

 setting free of cells which form a lining epithelium. Such formation 

 of leucocytes has been especially described in connection with the 

 lining epithelium of the ccelomic cavities, as stated in Chapter XII., 

 so that anatomists look upon the origin of these free cells as being 

 largely from the ccelomic epithelium, or mesothelium, as Minot 

 calls it. 



Then, again, the free cells which form the germinal cells can be 

 traced back to a germinal epithelium, which also is part of the cadorn. 

 Thus the suggestion arises that in the embryo a cellular lining is 

 formed to a ccelomic cavity (mesothelium) composed of cells which 

 have no communication with the nervous system, and are capable of a 

 separate existence as free individuals, either in the form of germinal 

 cells or of lymphocytes, hamiocytes, and plasma-cells, so that these 

 latter free cells may be considered as living an independent existence 

 in the body, and ministering to it in the same sense as the germ-cells 

 live an independent existence in the body. Again, the function of this 

 mesothelium apart from the germ -cell is essentially excretory and 

 phagocytic. It is the cells of the excretory organs as well as the 

 lymphocytes which pick up carmine-grains when injected. It is the 

 cells of the modified excretory organs, as mentioned in Chapter XII., 

 which, according to Kowalewsky and others, give origin to the free 

 leucocytes. 



We see, then, that the conception of a syncytial neuro-epithelial 

 host holding in its meshes a number of free cells leads directly to the 

 questions : What is the ccelom ? To which category does its lining 

 membrane belong? and further, also, What is the origin of these 

 free cells ? 



The Metazoa have been divided into two great groups — those 

 which possess a ccelom (the Ccelomata ; Lankester's Gelonioccela) 

 and those which do not (Ccelenterata ; Lankester's Enteroccela). As 

 an example of the latter we may take Hydra, because it is a very 



