890 The American Naturalist. [October, 
processes the ccelom grows toward the axial region, but never 
penetrates it, the primitive streak and head-process never devel- 
oping a ceelom. The changes which have taken place have now 
divided the mesoderm into two tissues: 1°, the mesothelium or 
epithelial lining of the body cavity; 2°, the mesenchyma com- 
prising all the non-epithelial mesoderm. Whether in all cases 
the ccelom begins as a series of small spaces, which subsequently 
fuse, we are unable to say; but it is my no means improbable 
that such is the case. It is, I think, also probable that the coelom 
begins always to appear at a little distance from the embryo, and 
spreads both centripetally and centrifugally. In the sheep the 
large size of the ccelomatic cavity is connected with the preco- 
cious development of the amnion. 
Of other vertebrates we can say only that the coelom appears, 
and is at first merely a narrow fissure. It divides the meso- 
derm into an upper leaf (Hautfaserblatt) and an inner or lower 
leaf (Darmfaserblatt); the former may be called the somatic, 
the latter the splanchnic mesoderm, as proposed by Balfour. 
The upper leaf lies close against the ectoderm; the two 
layers together form the somatopleur, or body wall. The 
lower leaf lies close against the entoderm; these two layers to- 
gether form the splanchnopleur, or wall of the alimentary tract. 
Both the somatic leaf of mesoderm and the splanchnic comprise 
mesothelium and mesenchym; axially the two layers become 
continuous, both with one another and with the axial mesoderm. 
The mesothelium continues for some time to throw off cells, 
which add themselves to the mesenchym, but except for this the 
two tissues have each an entirely separate history, and the adult 
tissues derived from them form two well-defined and natural 
groups. 
The morphology of the ccelom is so important that it is diffi- 
cult to understand why so many investigators have slurred over 
the question of its embryonic development. Exact observations 
on its first appearance and on the first stages of its expansion in 
various types are urgently needed, and would certainly do more 
than anything else to throw light on the still obscure problem of 
the origin of the mesoderm. 
