The Bird in the Egg 469 
living in the mud of the pond, which are the most lowly 
organized beings in the world. The gastrula stage—the 
double-walled cup, into which the real egg-part of each 
Fic. 366.—Third-day stage of embryo chick. (See Fig. 364.) Greatly enlarged. 
AM, Muscle-plates (false vertebree ). 
yolk forms itself, is comparable with the next higher class 
of living creatures, the sponges. For the simplest of 
these are nothing more than a cup of cells, two layers deep 
