No. 2.] THE MESODERM AND MESENCHYM. 363 
Crustacea or Turbellaria), and hence no high morphological 
value can be attributed to it. 
Obviously then the mesoderm, as well as the spaces enclosed 
or penetrated by mesodermal elements, cannot be granted impor- 
tance in morphological classification ; which deduction is in 
accord with a number of recent investigations, that stand in 
opposition to the acceptance of the germ-layer theory. For 
the development of the mesoderm and its cavities is, in its turn, 
dependent upon the cleavage and gastrulation processes. Thus 
a mesodermal pseudocoel as well as a coelom are usually blas- 
tocoelic; but the coelom may be also gastrocoelic in origin. 
Similarly there is no essential difference between formation of 
the mesoderm by detached and isolated cells, and by coelom 
sacks or epithelially united mesoderm stripes. 
Accordingly, I am led to conclude that the body cavities in 
different animal groups cannot be homologized merely on the 
ground of apparent similarity of development; for the earlier 
development and differentiation of these cavities must be 
referred, directly or indirectly, to the modes of cleavage and 
gastrulation, and the latter, as is well known, often differ widely 
in closely allied forms. 
The coelom of a vertebrate is frequently spoken of as being 
homologous with that of an annelid, since it passes through an 
apparently similar development. Now without stating or in 
any way wishing to imply that the homology of the coelom in 
this case is not correct, I would emphasize the point that the 
similarity of development is itself not an adequate reason for 
the homology. This standpoint should seem justifiable to any 
one acquainted with the facts reviewed in this paper, which 
tend to show how various the formation of mesoderm and its 
cavities are in closely related forms. 
Are, then, the body cavities possible of homologization ? 
Comparison of the modes of early development shows that the 
ontogeny is of little value in this connection; but it might be 
thought that comparative anatomy could be of avail in the 
search for homologies. 
But, though often spoken of as such, a “ body cavity” can- 
not be considered an organ, equivalent, e.g., to a brain or a 
