THE CELLULAR BASIS 17S 



jellyfish, starfish, worm, moUusk, insect or 

 vertebrate are foreshadowed by the character- 

 istic polarity, symmetry and pattern of the 

 cytoplasm of the egg either before or immedi- 

 ately after fertilization. In all of these phyla 

 eggs may develop without fertihzation, either 

 by natural or by artificial parthenogenesis, 

 and in such cases the characteristic polarity, 

 symmetry and pattern of the adult are found 

 in the cytoplasm of the egg just as if the lat- 

 ter had been fertilized. The conclusion seems 

 to be justified that these earliest and most 

 fundamental differentiations which distin- 

 guished the eggs of various phyla are not de- 

 pendent upon the sperm. 



All of these correspondences between the 

 polarity, symmetry and pattern of the egg 

 and of the developed animal are found in the 

 cytoplasm. It is possible that the polarity 



Fig. 42. Normal astd Inverse Symmetry ik Late Em- 

 bryos AND Adult Stages. In 1, cross-hatehed area is blasto- 

 pore; cells shaded by lines, mesoderm, other cells, endoderm; 

 the spiral twist of the snail begins in opposite directions in 

 the two embryos. In 2, the adult organization is shown with 

 all organs inversely symmetrical; os, olfactory organ; a, anus; 

 L, lung; V, ventricle; E, kidney. In 3, sinistral and dextral 

 shells of adult snails are shown. 



