532 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



primitive pit in late primitive-streak stages and later at the anterior end 

 of the head process. Frequency of eye development is higher in grafts from 

 the median than in those from the lateral regions of this field, and in those 

 from the left side than in those from the right, as in Rawles's experiments. 



Fig. i68. — Regional distribution of development of mesodermal structures in chorio-allan- 

 toic grafts of head-process blastoderm of chick; measurements as in Fig. 167; s, striated muscle; 

 H, nonstriated muscle (from Rawles, 1936; prepared by M'illier and Rawles). 



In later stages capacity for eye development becomes greater laterally 

 than in the median region. The higher frequency in median than in lateral 

 grafts of organs normally lateral, such as eye and ear, is like eye frequency 

 in amphibian development at certain stages and, like that, suggests prece- 

 dence of the median region in attainment of a certain physiological condi- 



