PHYSIOLOGICAL CIL\RACTERISTICS OF AXIATE PATTERNS 151 



ceptibility to developmental activity is evident in these teleosts, as else- 

 where. In Figure $2, B, C, K, and L, the region representing the dorsal 

 lip of the blastopore shows high susceptibility; in B and C stages are 

 figured showing the highest susceptibility in the region of early invagina- 

 tion. During elongation of the embryonic shield its anterior end becomes 

 the most susceptible region; but sooner or later, at somewhat different 

 stages in the different species, a second region of high susceptibility appears 

 posteriorly, as in other segmented animals. 



An electric-potential difference exists between apical and basal poles 

 in early stages of Fundulus development and undergoes cyclical reversal, 

 apparently in relation to cyclical changes in the cells. A potential differ- 

 ence also occurs along one axis of the blastoderm (Hyde, 1904). At the 

 time of maturation alkaline colloids accumulate apically, the blastodisc 

 becoming distinctly marked off from the acid remainder (Spek, 1933). 



AMPHIBIA 



The great volume of investigation concerned with amphibian develop- 

 ment in recent years has revealed many facts and perhaps presented even 

 more problems. We have learned something about the pattern of am- 

 phibian development, but the problem of pattern remains and continually 

 presents new aspects. Some of these are touched upon in later chapters; 

 the present concern is chiefly with some of the more direct evidence bear- 

 ing on the general characteristics of amphibian spatial or regional pattern 

 in earlier stages of development. 



The protoplasm-yolk gradient of the amphibian egg, the superficial 

 pigmentation present in most species except in the basal region of high 

 yolk content, and the gradient, decreasing basipetally, in rate of cleavage 

 indicate an apicobasal pattern. Dorsiventrality is clearly indicated in the 

 fertilized eggs of some anurans by the gray crescent, a less deeply pig- 

 mented, subequatorial region between the deeply pigmented apical, and 

 the light basal region, decreasing in width laterally and ventrally from 

 the mid-dorsal region. It is indicated in Figure 53, A-C. In eggs of other 

 amphibia a corresponding region is more or less clearly indicated by a 

 difference in appearance from other parts. Observations on differential 

 susceptibility, as indicated by cytolysis, give evidence of a definite pattern 

 of physiological condition undergoing progressive change with the prog- 

 ress of development. ■'' 



^' Bellamy, 1919; Bellamy and Child, 1924; material, Rana pipiens, Chorophihis nigritus 

 Bufo americaniis, and some data on Amblystoma tigrinum. 



