Vigr. 57. 



A 



a-Srt 



B 



i 



<( J 



m 



D^ 



1D rvi 



2T 





I 1^ 





I? 



3 K H V 



iffl 



\1 f*\'<: I ,| J W 



; 



tt 



F 



<* 







Fig. 57. — Successive stages of the development of the head of a chick. — /, IT, TIT, 

 first, second, and third cerebral vesicles ; T", vesicle of the cerebral hemispheres; /'', 



