470 The Bird 



(these layers being known as the ectoderm and endoderm, 

 or outer skin and inner skin) . The name gastrula, or Httle 

 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 geim-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 palaeontology, 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 — 



