894 - The American Naturalist. [October, 
mesothelium. The value of this theory lay in the clearness of 
its formulation, thus facilitating discussion, and also in its bring- 
ing out the difference more clearly between the epithelial and the 
non-epithelial portions of the mesoderm. As we have seen, there 
is no evidence of a character to render even probable that part ot 
the ccelom of vertebrates represents archenteric diverticula ; 
the whole mesoderm appears as a single germ-layer, which is 
subsequently differentiated into mesenchyma and mesothelium. 
Hence both essential parts of the coelom theory are inapplicable, 
at least in the present state of our knowledge, to vertebrates. 
For further discussion of the difficulties of the Hertwigs’ theory, 
see Rabl, 29, 198-202. The Hertwigs recognized the signifi- 
cance of the parablast, and added the important rectification, which 
Flemming’s observations had already rendered necessary, of sep- 
arating the smooth muscles from the striated skeletal muscles, a 
separation the propriety of which was wrongly questioned by 
Balfour (Comp. Embryol, II, 359). By this advance the two 
groups of mesodermal tissues became properly delaminated. 
C. Rabi's theory of the mesoderm is based, it seems to me, 
wholly upon his failure to understand the law of concrescence. 
That the mesoderm appears (perhaps in all vertebrates) while 
concrescence is going on is well ascertained; consequently there 
is an axial mesoderm (Rabl’s “ gastrules mesoderm”) where . 
concrescence has taken place, and a lateral mesoderm (Rabl’s 
“ peristomales mesoderm”) in the part of the blastodermic rim 
which has not concresced. Until Rabl proves that his “ peristo- 
males” mesoderm does not become axial mesoderm in later 
stages, his theory can have no standing. His memoir brings out 
one point of very great importance for the elucidation of the 
early stages of vertebrates, namely, that the “ peristomal ” meso- 
derm—in other words, that of the blastodermic rim in selachians 
and of the lips of the anus of Rusconi in amphibians—is repre- 
sented in the amniota by the mesoderm of the primitive streak. 
If this interpretation, which is much strengthened by L. Will’s 
researches on the Gecko, 44, be verified, then the primitive streak 
is the homologue in amniota of the anus of Rusconi, and is the 
region where concrescence is incomplete; the head-process is 
