130 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7 
The gastrula, present in the ontogeny of all animals, is the last 
structural complex which is of universal occurrence throughout the 
animal kingdom and the last common bond between the various animal 
types, and it therefore must in some way represent the starting point 
for all subsequent divergence. 
An egg typically divides into two, four and eight cells in three planes 
each of which is at right angles to the other two and, equal cell division 
continuing, a hollow sphere is formed, the blastula, which collapses, 
forming a more or less hemispherical structure with two layers of 
cells, an outer and an inner, the gastrula. 
The gastrula possesses a single axis which runs through the center 
of the opening resulting from the collapse of the blastula; but since 
the walls of the gastrula are everywhere the same there is a perfect 
radial symmetry about this axis. 
Since the gastrula, though usually in a considerably modified form, 
is common to all animal types and forms the starting point for the’ 
divergence of the various major groups, it is important to determine 
what its real significance is. 
From the egg through the blastula to and including the gastrula 
there is a direct geometrical continuity leading to the formation of a 
body radially symmetrical about a single axis. The logical termination 
of such development would be the formation of an animal type in 
which the gastrula axis persists to the adult and the body of the adult 
is radially symmetrical about it. 
If the facts presented by a study of embryology are significant in 
indicating the phylogeny of animals, it is clear that the last common 
ancestor of all the bilaterally symmetrical animal types was a radially 
symmetrical form, or a sort of adult gastrula. 
There are two such animal forms. In one of these, the sponges, the 
body consists of a community of cells imperfectly integrated and show- 
ing relatively little devision of labour or unified life. The sponges 
continue to grow throughout life, and their increase is always radial. 
In the other, the coelenterates, the body is a distinct unit of more 
or less definite size with a gastrovascular cavity and a well developed 
muscular system. Growth in the coelenterates as in the sponges is 
continuous throughout life; but since the complexity of the organiza- 
tion imposed a definite maximum size on the individuals, the growth 
impetus results in the continued formation of new individuals which 
bud off from those preceding, typically resulting in an arborescent or 
mass colony comparable to that seen in the phanerogams. While in 
many coelenterates the budded individuals become free, and in some 
