470 The Bird 
(these layers being known as the ectoderm and endoderm, 
or outer skin and inner skin). The name gastrula, or little 
stomach, is certainly most applicable, for an animal of 
this kind consists of hardly more than stomach and mouth. 
But the embryo of the frog’s egg does not long remain 
in this sponge-like condition; for almost immediately a 
third layer, the mesoderm, or middle skin, appears between 
the other two. From these three layers of cells all the 
parts of the body of the future chick arise, by the continued 
dividing of the cells. The details are far too involved to 
be followed without going into technicalities. 
Suffice it to say that in the development of the embryo 
chick we have one of the surest proofs of the truth of the 
theory of evolution,—of the gradual evolving of each of 
the higher groups of animals from some lower, more 
generalized form, until all are originally derived from an 
organism consisting of a single cell, with its tiny germ- 
spot. The dividing of this germ-spot in the dawn of 
creation was the beginning of that wonderful unrolling 
of life which to-day culminates in birds and the higher 
mammals,—even in man himself. 
It would be too much to expect that the growing 
embryo chick distinctly reflects in its successive stages of 
growth, during a short three weeks, the embryonic states 
of all its unnumbered generations of ancestors. The record, 
like that of paleontology, is imperfect. Many important 
phases are slurred over or apparently entirely omitted; 
in order, evidently, to give freer play to the development 
of organs which will be of vital importance in the future 
active life of the bird. Now and then, however, a gleam— 
