PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 149 



gional susceptibilities to the two agents was observed; the same pattern 

 appears with both. The relation between susceptibility and developmen- 

 tal activity seems evident. The higher susceptibility of the presumptive 

 dorsal side, and later of the actual dorsal side and the region about the 

 blastopore, indicates a change in condition of this region as gastrulation 

 approaches. This is of interest in comparison with the increase in rate 

 of reduction in the region corresponding to the dorsal lip of the blastopore 

 in the ascidian (Fig. 50, B, C) and the increase in susceptibility of the 

 amphibian dorsal region at gastrulation; it is also in line with the evidence 

 of higher rate of respiration and of dye reduction in this region (see 

 pp. 153-58). 



TELEOSTS 



Differential susceptibility of developmental stages of three teleost 

 fishes, Tautogolahrus adspersus, Fundulus heteroclitus , and Gadus morrhua, 

 has been determined by Hyman (192 1). Figure 52 shows some of the 

 stages. During early cleavage of Tautogolahrus susceptibility decreases 

 from center to periphery of the blastoderm {A). In the later blastoderm 

 the center is no longer the most susceptible region; disintegration begins 

 at the posterior border of the blastoderm, that is, in the region which is to 

 become the embryonic area, and proceeds anteriorly {B, C). This region 

 represents the dorsal lip of the blastopore, and its high susceptibility at 

 this stage corresponds with the findings in the ascidian, cyclostome, and 

 amphibian. After the embryo appears, the gradient in it is from anterior 

 to posterior and from dorsal to ventral {D, E). In still later stages the 

 posterior end of the embryo becomes a secondary region of high suscepti- 

 bility, and death progresses anteriorly from it, as well as posteriorly from 

 the head region and laterally and ventrally from the dorsal region {F). 

 In its earlier stages Fundulus resembles Tautogolahrus, but in the embryo 

 the posterior region of high susceptibility appears earlier and, up to ad- 

 vanced embryonic stages, is more susceptible than the anterior end (G-/). 



The development of Gadus shows somewhat different susceptibihty re- 

 lations in earlier stages (Fig. 52, J-P). In this species the germ ring de- 

 velops early, and it and the embryonic shield are much more distinct and 

 more sharply marked off from other parts than in the other two forms. In 

 the earliest stages observed, the periphery of the blastoderm where the 

 germ ring is forming is more susceptible than the center (/). In early 

 germ-ring stages one side of the ring is more susceptible than the other; 

 and when the embryonic shield forms, it is the most susceptible region 

 {K, L). In more advanced stages of the shield and in early stages of the 



