SOME TEACHINGS OF DEVELOPMENT. 211 
surface between the cells of the general layer of ectoderm, which 
covers the under surface of the umbrella, and they evidently 
belong to that layer. In other words, the deeper parts of some 
of the ectoderm cells are modified to form the muscular struc- 
tures. In Hydra, according to the descriptions of Kleinenberg 
and Korotneff, this is still more clearly the case — in fact, almost 
all the large superficial cells of the ectoderm are thus modified 
in their deeper part, although the muscular development is less 
characteristic. Now, supposing these muscular cells of the 
jelly-fish, in place of remaining amongst the rest of the ectodermal 
cells, to sink deeper into the surface of the jelly, or even to 
become imbedded in its substance, they would then appear 
altogether distinct ; and although originally derived from the 
ectoderm, would be reckoned as part of the mesoderm. This is, 
indeed, the condition which is actually found in many other 
Coelenterates. 
We have seen that in the lower forms of the Metazoa the 
cells of the intermediate layer are derivable from almost any 
part of the primary layers. But in all the higher forms the 
mesoderm is developed from one part only of those layers, and 
this part is very frequently close to the contracted orifice of the 
gastrula, at the place where ectoderm and entoderm pass round 
into one another.^ And from this place of origin the mesodermic 
cells — consisting of those which are to minister to support, and 
those which are to minister to movement intermixed— spread out 
between the two primary layers. We see a typical instance of 
this in Paludina and also in Unio. Now, if we suppose that the 
segregation appeared first at one particular point at the margin 
of the protostome, and afterwards spread in all directions from 
that point, we can comprehend how bilateralism might have 
appeared as the result of the separation of mesoderm cells (acci- 
dentally ?) at one part only of the margin of the protostome. 
But whatever might have been the original conditions, the meso- 
derm as we now actually see its development, appears from the 
first in two halves, and bilateralism makes its appearance simul- 
taneously with these. 
Our third stage, then, such as we see it in all animals above 
the Coclenterates, — the segregation, namely, of an intermediate 
If we agree with Haeckel in surmising that in the earliest stage of 
animal evolution in which the cup-form or gastrula appeared, the oriGce 
functioned as a mouth, we can readily understand how it is that a 
general mesodermic cell-segregation should have tended to localise itself 
in the neighbourhood of that aperture. For this would probably, earlier 
than other parts, become the seat of special active functions; e.g. the 
opening and closing of the orifice or even the sucking in of aliment, such 
as we see occurring in the early embryo of the earth-worm. 
