EMBRYONIC RECONSTITUTIONS 



531 



and lateral areas but in higher percentage in median (brain parts, eye, 

 ear, mesonephros) . The highest frequency of graft development is in 

 median pieces including head process and node, but pieces from the left 

 side show a higher frequency than those from the right. Development of 



P'iG. 167. — Map showing regional distribution of development of ectodermal structures in 

 chorio-allantoic grafts of the head-process blastoderm of chick; distances from the primitive 

 pit indicated in millimeters are average measurements (from Rawles, 1936; prepared by 

 Willier and Rawles). 



the pieces indicates an anteroposterior and mediolateral physiological pat- 

 tern of some sort, in relation to which different developmental potencies 

 are realized. 



A further study of eye development in chorio-allantoic grafts of late 

 primitive-streak and head-process stages has shown that the potentiaHty 

 for eye development is realized in the grafts from an area of field about the 



