ORIGINS OF EMBRYONIC PATTERNS 657 



of the egg, and polarity of the embryo there is definite and characteristic 

 relation of each step in the development of pattern to the pre-existing 

 pattern. The data suggest that polembryony is associated with alteration 

 of the physiological relations that determine the usual sequence of events. 



POLARITY OF ANIMAL EGGS IN RELATION TO THEIR ENVIRONMENT 



The chief axiate pattern of animal eggs, so-called "polarity," may be 

 morphologically evident in a graded distribution, or a more or less sharply 

 defined separation of cytoplasm and yolk or other substance, in position 

 of nucleus or region of polar body formation, or it may be independent of 

 all these. In some eggs it is apparently present or, under experimental 

 conditions, persists only in the superficial cytoplasm or cortex; in others 

 it may appear as a graded distribution of materials throughout the egg; 

 in some eggs it is perhaps not estabHshed until the polar bodies form. 



Ventrodorsality or dorsiventrality may also be visible in some eggs, at 

 least after fertilization— for example, in the mesoplasm of the Styela egg 

 (p. 577) and the gray crescent of the frog egg — and often, when it is not 

 visible, experiment indicates its presence even before fertihzation. In 

 some forms, however, ventrodorsality becomes evident only in the course 

 of development; and experiment does not give conclusive evidence of its 

 presence in the unfertilized egg, though some physiological basis for it, 

 presumably a slight regional difference of some sort, is probably present. 

 This seems to be the condition in the sea-urchin egg. In ascidian eggs 

 without visible dorsiventrality development of isolated egg pieces gives 

 evidence of a dorsiventral pattern. The specific asymmetries of later 

 stages of various animals are not usually evident in undivided eggs with 

 methods at present available, though difference in developmental be- 

 havior may indicate that difference of some sort is present. 



All lines of evidence indicate that patterns of animal eggs are relatively 

 simple as compared with later stages, that is, they represent the most gen- 

 eral features of developmental pattern. This general pattern apparently 

 constitutes a sort of physiological co-ordinate system in relation to which 

 further development of pattern takes place. 



Relations of the developing oocyte to its intraorganismic environment 

 differ widely in different animals, and the question of conditions in the 

 oocyte environment as possible factors in determination of its pattern is 

 best considered according to the character of this relation.* 



« In an extended description of oocyte development in invertebrates Korschelt and Heider 

 (1902) distinguish diffuse and localized egg formation and in both of these types, solitary 



