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 



J^rain 



Eyt 





Heart 



Qills 



V-': ff'-'^^-i^^i^m 



\ ■•■>::-s: • I 'i«;v >?"''*■■••■■• ■••"< 



..■:.- J:y^^;^rm}?m--'' -'-'sy'-'' 



•■ "T*,-. ;~v 'y** ,■.■•••.■.•• •.• 



''V*^» 



* *Vi*... 



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Fig. 366.— Third-day stage of embryo chick. (See Fig. .361.) Creatly enlarged- 



M, Muscle-plates (false vertehne). 



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 



