I9 2 Heredity and Environment 



may occur on the right side instead of the left, the pyloris and 

 chief portion of the liver on the left instead of the right, etc. 

 Among certain snails this inversion of symmetry may occur 

 regularly in certain species and not in others, the inverse form 

 being known as sinistral and the ordinary form as dextral (Fig. 

 72). In these sinistral snails, and probably in all animals show- 

 ing inverse symmetry, the embryo is inversely symmetrical and 

 every cleavage of the egg from the first to the last is the inverse 

 of that which occurs in dextral snails (Figs. 70-72). There is 

 good reason to believe that in such cases the unsegmented egg is 

 also inversely symmetrical as compared with the more usual type 

 (Fig. 70). In all of these cases there is a direct correspondence 

 between the polarity and symmetry of the oosperm and the polar- 

 ity and symmetry of the developed animal (Figs. 68-72). 



(d) Localisation Pattern. — In many animals the ectoderm and 

 mesoderm may be traced back to areas of peculiar protoplasm in 

 the oosperm, but, in addition to this, one can recognize in the as- 

 cidian 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 posi- 

 tions and proportions which they will have in the embryo and 

 larva (Figs. 10, 11, 46-48). 



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 echinoderm, the an- 

 nelid-mollusk and the chordate types of cytoplasmic localization 

 (Fig. 73). The polarity, symmetry and pattern of a jellyfish, 

 starfish, worm, mollusk, insect or vertebrate are foreshadowed by 

 the characteristic polarity, symmetry and pattern of the cyto- 

 plasm of the egg either before or immediately after fertilization. 

 In all of these phyla eggs may develop without fertilization, 

 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 latter had been 

 fertilized. The conclusion seems to be justified that these earliest 



