220 
BIOLOGY: A. H. CLARK 
Proc. N. a. vS. 
The hollow sphere with its increasingly thin walls due to repeated cell 
divisions becomes mechanically unstable and collapses, forming a cup with 
a two layered wall, the inner lining being composed of the inturned cells 
of the collapsed portion. 
There is now a differentiation of the cells into an external and an internal 
layer, but each of the cells in both layers is the equivalent of all the others 
in the same layer. A definite axis has appeared, passing through the 
center of the mouth of the cup (the gastrula opening) and through the 
opposite pole. 
Since in every plane passing through the central axis the body wall is 
just the same as it is in every other plane the continued development of 
such a typical gastrula would result in the formation of a radially symmet- 
rical animal developed about the axis of the gastrula as a center. 
The logical conclusion is that those animals which develop radially 
about the original axis are biologically the most perfect of all animals since 
they continue without interruption into the adult stage the original de- 
velopmental plan. 
There are two groups of animals which develop in this way. In one of 
these, the sponges, the larva grows into a sort of radial mass colony or 
imperfectly integrated community of cells with little division of labor or 
unified life and no definite organs. In the other, the coelenterates, the 
gastrula grows into an adult which resembles it in all its essential features, 
though innumerable minor refinements are added. 
The sponges, being without any definite structures, indeed little more 
than proliferated masses of slightly differentiated cells, increase in size 
by simply growing out in all directions from the main axis and usually also 
upward. 
The coelenterates retain the definite body form of the original gastrula 
which with increase in size rapidly weakens mechanically in spite of the 
development of a series of internal buttresses; furthermore, they have 
definite organs, such as tentacles, which must be supported by the body 
wall, and they must be strong enough to capture and to hold the organisms 
which serve them as food. They are therefore limited to a definite and 
relatively small body size, excepting in a few cases of extreme specializa- 
tion. But just as in the sponges there is no definite limit to the original 
growth impetus. In response to this the original polyp, having reached 
its full size, gives off a bud which grows into another polyp and this process 
may be continued indefinitely so that enormous arborescent or other 
colonies are formed. 
Both the sponges and the coelenterates continue in the same way to in- 
crease in bulk throughout life, the former by simple augmentation of the 
mass as a whole, the latter by the indefinite reduplication of the original 
unit. The colonial habit is a fundamental attribute of all coelenterates, 
