PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 163 



neural folds is more susceptible than other parts of the head; and as 

 closure progresses, the region of high susceptibility moves posteriorly and 

 then disappears. As the optic lobes develop, they become temporarily 

 more susceptible than other parts of the head, and later the otic primordia 

 also appear as regions of high susceptibility. Preceding torsion, the hind- 

 brain region again becomes highly susceptible, a change probably associ- 

 ated with torsion and disappearing later. Susceptibility becomes high in 

 the primordium of the heart, decreasing anteriorly from the posterior end, 

 where the sinus, the dominant region of the heart, finally develops (Hy- 

 man, 1927&). As the tail bud develops, high susceptibility appears in it, 

 and the limb buds become highly susceptible as they appear. In general, 

 except where lateral organs become localized regions of high susceptibility, 

 the embryo shows decrease in susceptibility from the median region lat- 

 erally. The remarkable correspondence of regions of high susceptibility 

 to agents as different in their action as potassium cyanide, ammonia, 

 sodium hydroxide, and ultra-violet light with regions of rapid dye reduc- 

 tion and, so far as observations go, with regions of high susceptibility to 

 Janus green is highly significant not only as evidence of the changes in 

 physiological pattern as development proceeds but also as indicating that 

 differential susceptibility shows certain real physiological differentials and 

 their changes. The results suggest that activation is an essential factor, 

 in the early stages of development, not only of the whole embryo but of 

 particular regions and organs. 



MAMMALS 



There is, at present, no direct evidence bearing on the question of 

 physiological pattern in early mammalian development, but much indirect 

 evidence from the occurrence of teratological forms, many of which are 

 similar to the differential modifications of development under experimen- 

 tal conditions in other vertebrates described in the following chapter. 

 Moreover, there is adequate ground for the belief, very generally held, 

 that there are fundamental similarities in developmental pattern among 

 vertebrates and that the differences in course of development are, in large 

 part, dependent on presence and amount of yolk in fishes, amphibians, 

 reptiles, and birds and its absence, together with uterine development, in 

 mammals. 



Functional patterns of various organs of adult mammals indicate a 

 gradient pattern of some kind as an underlying factor. The heart with 

 its dominant region at the sinus end, the high end of a gradient in early 



