12 LIBBIE H. HYMAN. 



In addition to this general double gradient, special regions of 

 high susceptibility appear temporarily during development. The 

 most important of these are: all places where folds, unions, or 

 bendings are about to occur; and the special sense organs. 



In general any extensive developmental change is preceded by 

 an increased susceptibility, that is, an increased metabolic rate 

 in the group of cells involved. Intense metabolism thus appears 

 to be a necessary condition for active development. 



II. APPLICATION TO NORMAL DEVELOPMENT. 



The foregoing observations may throw some light on problems 

 of amniote development and on the physiological conditions 

 underlying normal and teratological development. In utilizing 

 these observations for the interpretation of development, the 

 susceptibility differences described in the preceding section will 

 be regarded as representing differences in the rate of cell activity, 

 the most active regions being the most susceptible to lethal 

 solutions. 



The primitive streak is described in textbooks of embryology 

 as an undifferentiated linear ectodermal thickening. The im- 

 portant researches of Hoadley ('26, a, b, c), however, have shown 

 that there is some localization of developmental potentialities in 

 blastoderms younger than the typical primitive streak stage, 

 blastoderms of four to eight hours incubation. My observations 

 on the typical primitive streak stage (about fifteen hours incu- 

 bation) indicate that there is also present a physiological grada- 

 tion along the streak. The anterior end of the streak is the seat 

 of a high rate of activity, this rate diminishing posteriorly along 

 the streak. My observations of course do not show that this 

 physiological differentiation precedes the morphological localiza- 

 tion for the investigations of Hoadley concern earlier stages 

 than those on which I was able to observe reliable susceptibility 

 differences. But it must be recalled that in the hen's egg the 

 direction and position of the primary antero-posterior axis of 

 the future chick are already established in the ovocyte, being 

 determined by the attachment of the follicle in the ovary. 

 In the pigeon's egg also Bartelmez ('12) showed that the main 

 axis is determined in the ovocyte and is evidenced during the 



