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 



^ --^^^ ^■V^s*.:^ '!t'-^'4^ ^ 



(7;//^ 



"■ .v<:v'- ,>^-i^?^?i>f:•,r^^,v.-•■ 







.•'/ 



.'1 





Fig. 366. — Third-day stage of embryo chick. (See Fig. 364.) Greatly enlarged- 

 M, Muscle-plates (false vertebrse). 



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 



