534 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



of the streak. It appears probable, however, that m many of the experi- 

 ments full realization of developmental capacities may be limited or in- 

 hibited by the experimental conditions. The chorio-allantoic grafts of the 

 smaller blastoderm pieces do not give rise to orderly definite embryos of 



Fig. 170. — Map showing by shading certain organ areas of chick at head-process stage, 

 as defined by chorio-allantoic grafts. In the median region at the anterior end of the head proc- 

 ess is the eye area; heart develops from the large lateral areas; between them and median is 

 the mesonephric area and central within it a cross-lined area representing the adrenal-gonad 

 area; depth of shading in the various areas represents roughly intensity or degree of develop- 

 mental potency for the organ concerned under the conditions of experiment (prepared by Wil- 

 lier and Rawles, kindness of Dr. Willier). 



small size or even to axiate partial forms, but to tissue complexes; and 

 many of them fail to develop at all, though they presumably possess the 

 same potencies as others that do develop. Such failures may, of course, be 

 due to incidental conditions, injury in isolation and grafting, inadequate 

 attachment, inadequate blood supply, etc. But other factors are appar- 

 ently concerned. Even when whole blastoderms of primitive-streak, head- 



