172 HEREDITY AND ENVIRONMENT 



such cases the unsegmented egg is also in- 

 versely symmetrical as compared with the 

 more usual type (Fig. 40). In all of these 

 cases there is a direct correspondence between 

 the polarity and symmetry of the oosperm 

 and the polarity and symmetry of the devel- 

 oped animal (Figs. 38-42). 



(d) Localization Pattern. In many ani- 

 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. 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 



