588 PATTERNS AND PROBLEMS OF DEVELOPMENT 



lowed by normal or almost normal development. With extreme degrees 

 of stratification in the longitudinal axis of the beetle eggs, a dwarf larva 

 develops from the plasmatic part, the centrifugal yolk mass remaining 

 undeveloped or including some embryonic tissue without definite pattern. 

 At the blastoderm stage and later, centrifuging has little effect in altering 

 distribution of substances. Normal, or almost normal, embryos may also 

 develop from centrifuged muscid eggs, but with sufficient centrifuging the 

 embryonic zone is intermediate between a still lighter centripetal and a 

 heavier centrifugal zone; these may remain without development or be 

 taken into the embryo. 



The cortical cytoplasm in which the blastoderm forms after nuclei mi- 

 grate into it from the interior of the egg may be displaced toward one or 

 the other end of the egg, but the fact that the embryos developing from 

 centrifuged eggs show normal axial orientation indicates that the original 

 pattern persists. Moreover, with centrifuging beginning in stages when 

 few nuclei are present, these may be displaced centripetally so that only 

 the centripetal part of the plasmatic zone becomes nucleated, and blasto- 

 derm and embryo are limited to this region. This is apparently develop- 

 ment of a whole pattern from a part of the original pattern, that is, a 

 reconstitution, and suggests, as do the other reconstitution experiments 

 with insect eggs, that pattern at this stage may be little or nothing more 

 than a quantitative differential. 



It is sufficiently evident from the data mentioned that developmental 

 pattern in many eggs is, to a high degree, independent of stratification of 

 visible cytoplasmic constituents or inclosures. Alterations of cleavage by 

 centrifugal force may be due to displacements of nucleus of spindle or to 

 mechanical obstacles to cleavage resulting from aggregation of yolk. Ob- 

 viously, the stratified substances are, in most cases, not "formative sub- 

 stances," and formative pattern is independent of their distribution. This 

 pattern, then, must be a property of the "ground substance" (F. R. Lillie, 

 1909), supposedly not displaced by centrifuging; or it must be localized 

 in the cortex or superficial cytoplasm of the egg, in which displacement or 

 stratification of substances apparently does not result from centrifuging. 

 Differential susceptibility and differential dye reduction seem to indicate 

 that, in at least some eggs, a polar gradient pattern is present throughout 

 the egg before the cytoplasmic movements associated with maturation 

 and fertilization begin. If this is the case, this pattern perhaps persists 

 only in the superficial cytoplasm, which is less, or not at all, involved in 

 the streaming of deeper layers or in the stratification by centrifugal force. 



