584 



half would be changed, but the relative positions of the materials in each 

 half would be unchanged. This would indeed lead to inverse types 

 of symmetry but not such as are found in nature, where the inverse 

 form is the mirrored image of the normal one. In sinistral gaster- 

 opods not only is the adult the mirrored image of dextral forms but 

 the cleavage is also and it is probable that the unsegmented egg of 

 a sinistral snail is the mirrored image of the egg of a dextral one. 



By what changes may a „dextral egg" be converted into a „sini- 

 stral" one? In attempting to formulate an answer it must be borne 

 in mind that the change must be one which can readily take place, 

 since in the same species some individuals may be dextral and some 

 sinistral. Furthermore the answer to this question should be capable 

 of explaining all cases of inverse symmetry since the inverse symmetry 

 of snails is but a specific instance of a general phenomenon. Such 

 considerations led me to the view that the true explanation of this 

 reversal of symmetry is to be found in the reversal of the polarity 

 of the egg. Any one who has studied egg cells, or who has ever 

 given any attention to the matter, knows that one gets a mirrored 

 image of the structures at the animal pole if he views them from the 

 vegetative pole through the transparent egg; of course the same is 

 true of the vegetative pole and of all parts between the two. If, 

 therefore, the polarity of the egg were reversed it would bring about 

 a reversal of symmetry. Such reversal of the polarity of the egg 

 could occur in the ovary, the free pole of a dextral egg corresponding 

 to the attached pole of a sinistral one, and for some time I was 

 inclined to think that this actually occured, but au extended study 

 of the ovarian eggs of Limax, Polygyra, Succinea, Limnaea, Physa, and 

 Planorbis has shown that the polarity of the fully formed ovarian egg 

 is the same in all of these cases. Whether the animal be dextral or 

 sinistral the nucleus is a little nearer the free than the attached pole, 

 Fig. 1, though after the egg has been set free it again comes to lie 

 nearer the center ; the centrosomes usually if not invariably arise in 

 an indentation of the nucleus next to the free pole and the cell body 

 at this pole consists of relatively more cytoplasm and less yolk than 

 at the attached pole. 



The reversal of polarity does not, therefore, occur in the ovary, 

 but only after the eggs are set free. Unfortunately it is usually 

 impossible to distinguish one pole of a free egg from another before 

 the formation of the polar bodies, and, for all that direct observation 

 teaches, such an egg might be, in truth, apolar. We know however 



