1890.] The Mesoderm and the Calom. 879 
The fact that the mesoderm appears first in the embryonic rim 
renders it easy to make sure of its springing from the entoderm. 
Later, when concrescence moves the rim into the axial line, all 
three germ-layers are united in the axis of the primitive streak, 
and it begomes more difficult to decide which of the layers the 
mesoderm is specially connected with. To conclude: In Elasmo- 
branchs the mesoderm arises over alimited area of the entoderm 
near the ectental line; it separates from the entoderm apparently 
by a process of delamination, but the exact means of separation 
have yet to be investigated; it remains for a while connected 
with the entoderm along the embryonic axis; after its separation 
from the entoderm the mesoderm expands by proliferation of its 
own cells, and receives no accretions from the yolk, so far as at 
present known. 
Mesoderm of Teleosts—So far as the published accounts go 
the middle layer of bony fishes arises, as maintained by Balfour 
(Comp. Embryol., II., 74), from the entoderm. Such appears to be 
the significance of Ryder’s observation, 34, 41, of A. Goette’s, 9, 
E. Ziegler’s, 45, Agassiz and Whitman’s, 7, and of others. Fora 
good description, together with citations of conflicting authorities, 
see M. Kowalewski, 26, 469-474. Apparently the blastodermic 
rim is turned under, and the turned-under portion yields the 
entoderm, and is intimately connected with the sheet of meso- 
dermal cells, very much as in sharks; the mesoderm is several 
layers thick, and extends under the ectodermal blastoderm, 
gradually thinning out; the cells of the middle layer are at first 
closely compacted. | 
Mesoderm of Amphibia —Here it seems also clearly established 
that the mesoderm arises from the entoderm, principally along 
and alongside the median line, as a sheet of cells with no cavity 
(ccelom) included between them; along the axis of the primitive 
streak and at the blastoporic margin the connection between the 
mesoderm and the entoderm is both evident and intimate (see 
Bellonci, 5, Tav. 11., for figures showing this point in the axolotl, 
and O. Schultze, 35, for similar figures of Rana fusca). These 
facts have been recorded by so many observers that there can be 
little doubt or none of their entire accuracy (see the description 
