472 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 hemocytes 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 XIL., 
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 ccelom. 
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, hemocytes, 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 celom? 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 Ccelomoccela) 
and those which do not (Ccelenterata ; Lankester’s Enteroccela). As 
an example of the latter we may take Hydra, because it is a very 
