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 



Ear 



Heart 



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QiUs 



Tif :^ ■<^''' , 





fr 





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 • ■•■ ■.■^;v"^^■■^•**S?'■ :■•'■;.:•?/ ■•";■.• 



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

 M , Muscle-plates (false vertebrae). 



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 



