RECONSTITUTIONAL PATTERNS IN EXPERIMENT 377 



of section." These results give little evidence of any pattern of localized 

 substances or regional cytoplasmic differentiation. Even polarity is al- 

 tered by section, according to these authors, the surface of section becom- 

 ing the basal region. With isolated pieces of unfertilized and fertilized 

 eggs of Arhacia somewhat different results are obtained by Harnly (1926). 

 Localization of micromere material between nucleus and center of egg is 

 indicated, and micromeres do not form on the surface of section. Accord- 

 ing to Plough (1927, 1929), development of isolated 1/2 blastomeres of 

 Arhacia, Echinarachnins, Echinus, and Paracentrotus indicates that micro- 

 mere material is localized at the time of first cleavage; but neither this 

 localization nor that postulated by Harnly agrees with the results of 

 Taylor, Tennent, and Whitaker. According to results obtained with other 

 species, polarity is not altered by section.'-' 



These various results cannot, at present, be brought into agreement, 

 and it is not likely that all of them represent species differences. Perhaps, 

 however, if the polar pattern of the sea-urchin egg is primarily a quantita- 

 tive dynamic gradient system rather than a pattern of localized specific 

 substances, the different results may prove less difficult to reconcile. The 

 high temperature of the water in which Lytechinus lives (Tortugas) may 

 conceivably account for the apparent susceptibility of the polar pattern 

 to section. As regards development of micromeres from the surface of 

 section in Lytechinus, it has been found that in Strongylocentrotus and 

 Dendraster the micromeres appear at the low end of the primary polar 

 dye-reduction and susceptibihty gradients. They, together with prospec- 

 tive entoderm, apparently undergo an activation preceding gastrulation 

 (pp. 134-40). In the light of these data it appears possible that micro- 

 meres might form on the surface of section in consequence of a depression 

 resulting from the injury to the cell. 



Sectioning sea-urchin eggs or early embryos in or near the frontal plane 

 results in reversal of ventrodorsality in the dorsal but not in the ventral 

 portion. Ligaturing in a meridional plane may determine ventrodorsality 

 vertical to the plane of ligature. With ligature tightly drawn, two op- 

 posed ventrodorsalities result, dorsal sides of both being toward the liga- 

 ture. '^ If the ventral side is the high end of the ventrodorsal gradient, 

 as it appears to be, there is apparently depression or inhibition in the plane 



" Taylor and Tennent, 1926; Taylor, Tennent, and Whitaker, 1926; Tennent, Taylor, and 

 Whitaker, 1929. 



'3 H5rstadius, 1936(7, b, 1937; Horstadius und Wolsky, 1936. 



'•< Horstadius, 1936a, h, 1937, 1938; Horstadius und Wolsky, 1936. 



