PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 153 



and ventrally over other parts. The region of the dorsal Hp of the blasto- 

 pore is more susceptible than the apical pole in the early gastrula; from 

 this region disintegration extends anteriorly to a greater extent than in 

 earher stages, but also extends basipetally from the apical pole (Fig. 53, /)). 



The anterior end of the embryo is localized near, or more or less ventral 

 to, the apical or animal pole of the egg, according to the species; the apical 

 region, then, becomes anterior. After gastrulation is completed, two areas 

 of high susceptibility still persist, one anterior, the other dorsal; but the 

 dorsal area is less susceptible than the anterior. In the earlier neurula 

 stages disintegration begins in the median anterior region of the neural 

 plate; and in slightly later stages, usually at two points where the optic 

 primordia are developing. From this anterior region it spreads posteriorly 

 along the neural plate and joins the area of disintegration at the posterior 

 end of the embryo. Susceptibility of neural-plate stage to radium shows 

 essentially the same differentials (Stachowitz, 1914). In early stages of 

 organ development optic vesicles, nasal pits, ventral suckers, and tail bud 

 all appear as local areas of high susceptibility with local radial or axial 

 gradients. 



The data on susceptibility indicate that the dorsal region is different 

 in some way from lateral and ventral regions, but the apical region is 

 primarily the most susceptible. As development progresses, the relative 

 susceptibility of the region which is to become the dorsal lip of the 

 blastopore increases, until at the beginning of gastrulation it is the most 

 susceptible part of the embryo, and in it susceptibiHty decreases anteriorly 

 and laterally from the blastopore. This is the region which, after invagina- 

 tion, becomes the so-called ''organizer" or "organization center" (see 

 pp. 454-70). In a study of differential susceptibility in other amphibian 

 species Cannon (1923) found practically no uniformity in the suscepti- 

 bility of different regions at various stages of development (see Appendix 



VI, p. 743)- 



Recent investigations on the respiration of the dorsal lip region, as 

 compared with other parts, are of much interest in relation to the role 

 of this region as inductor of the neural plate and to the data on suscepti- 

 biHty. In thirty-six out of forty-four cases (82 per cent) in which the 

 dorsal lip region was destroyed, oxygen uptake was found to be lowered 

 by 30 per cent or more, as compared with controls in which an equivalent 

 destruction of an indifferent region had been accomplished. Gastrulae 

 from which the dorsal lip region has been removed have a lower CO^ 

 production than those from which an equal volume of cells has been re- 



