stage having even more striking resemblances to worms. All the larvæ of
insects are therefore like one another, and like worms also, in certain
fundamental characters of internal and external structure; so the
conclusion that the whole group of insects has arisen by evolution from
more primitive ancestors resembling the worms of to-day is based upon
mutually explanatory details of comparative anatomy and embryology.

* * * * *

Let us now turn back to some of the earlier pages of the embryological
record which we passed over in order that we might translate the later
portions dealing with more familiar and intelligible structures like
gills. Before the egg of the frog becomes an elliptical mass of cells, it
is at one time a double-walled sac enclosing a central cavity; in this
stage it is called a _gastrula_. Tracing back the mode of its formation,
we find that it is produced from a hollow sphere of fewer cells that are
essentially alike; this stage also is so important that the special term
_blastula_ is applied to it. Still earlier, there are fewer cells--128 or
thereabouts, 64, 32, 16, 8, 4, 2, and 1. In other words, the starting
point in the development of the frog is a _single biological unit_; this
divides and its products redivide to constitute the many-celled blastula
and the double-walled gastrula. All the other animals we have mentioned
begin like the frog, as eggs which are single cells and nothing more; they
too pass on to become blastulæ and gastrulæ, similar to those of the frog
in all essential respects, particularly as regards the nature of the
organs produced by each of the two primary layers, and the mode of their
formation. Does the occurrence of blastulæ and gastrulæ and one-celled
beginnings mean that the higher animals composed of numerous and much
differentiated cells have evolved in company from two-layered saccular
ancestors which were themselves the descendants of spherical colonies of