PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 147 



caudal reduction is relatively less than earlier. Before resorption of the 

 tail begins, its reduction gradient becomes very slight; during resorption 

 traces of it may persist, or only irregular areas of more rapid reduction 

 scattered along the tail appear (Fig. 50, H). As this figure indicates, the 

 more rapid reduction on the neural side of the larval body persists. 



The appearance at the gastrula stage of a region of high rate of reduc- 

 tion corresponding to the dorsal inductor region of amphibia, the dorsal 

 lip of the blastopore, is of interest, as compared with the higher rate of 

 respiration and oxidation in this region of the amphibian gastrula ob- 

 served or indicated by most experimental data (pp. 153-58) and in rela- 

 tion to the view, now widely held, that there are fundamental similarities 

 in spatial embryonic pattern throughout the chordates. Differential per- 

 manganate reduction does not, of course, give any information as legards 

 presence or absence of regional cytoplasmic specificities. It does suggest, 

 however, that a quantitative gradient pattern perhaps underlies and pre- 

 cedes the regionally specific pattern. 



CYCLOSTOMES 



Differential susceptibility to ammonia and acetic acid of the lamprey 

 (Entosphenus appendix), as determined by Hyman (19266), is shown in 

 Figure 51. The susceptibility gradient of the unfertilized egg (A) and 

 early cleavage stages (B, C) is basipetal. In more advanced cleavage it 

 is in general basipetal; but some irregularities as regards cells or cell 

 groups appear, perhaps because susceptibility differs at different stages 

 of the division cycle (D). In the early blastula the basipetal gradient is 

 distinct and symmetrical (E); but in the later blastula one side, pre- 

 sumably dorsal, becomes more susceptible than the other {F, G). The 

 region of invagination is more susceptible than any other part in the early 

 gastrula {H), the dorsal side is more susceptible than the ventral (/, /), 

 and this dorsiventral difference increases as gastrulation proceeds {K, L). 

 As the neural groove develops, its anterior region becomes the most sus- 

 ceptible part of the embryo {M), but the original basipetal (anteropos- 

 terior) gradient and the higher susceptibility of the blastopore region are 

 still evident in the later stages of disintegration (A^) . With elongation of 

 the head region death progresses from the anterior end of the brain pos- 

 teriorly in the dorsal region and laterally and ventrally from this region, 

 and a region extending dorsally and anteriorly from the blastopore also 

 shows high susceptibility (0, P) ; this, according to Hyman, is the segmen- 

 tal plate or somite-forming region. At the time of hatching, the gradient 



