THE CELLULAR BASIS 181 



mals the ectoderm, endoderm and mesoderm 

 may be traced back to areas of peculiar proto- 

 plasm in the oosperm, but in addition to this 

 one can recognize in the ascidian egg areas of 

 peculiar protoplasm which will give rise to 

 mesenchyme, muscles, nervous system and 

 notochord, and these substances are present 

 in the oosperm in the approximate positions 

 and proportions which they will have in the 

 embryo and larva (Figs. 9, 10, 27-29). 



Indeed there are types of localization of 

 these cytoplasmic materials in the egg which 

 are characteristic of certain phyla; thus there 

 are the ctenophore, the flat-worm, the echino- 

 derm, the annelid-mollusk and the chordate 

 types of cytoplasmic localization (Fig. 43). 

 The polarity, symmetry and pattern of a 

 jellyfish, starfish, worm, mollusk, insect or 

 vertebrate are foreshadowed by the character- 



Fig. 42. Normal and Inverse Symmetry in Late Em- 

 bryos and Adult Stages. In 1, cross-hatched 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; K, kidney. In 3, sinistral and dextral 

 shells of adult snails are shown. 



