586 



from one pole to the other can be accepted as satisfactory proof that 

 this movement does occur. Nevertheless, indirect evidence makes it 

 not improbable that the reversal of the polarity of the egg occurs in 

 the stage and manner already described. This is indicated by the 

 fact that such a reversal entirely and satisfactorily explains all the 

 phenomena of inverse symmetry in embryonic and adult stages, whereas 

 no other explanation does this even approximately ; no other accounts 

 for the fact that such apparently profound changes in organization 

 may occur as mere individual variations which have no phylogenetic 

 importance; no other is generally applicable to all cases of inverse 

 symmetry wherever found. 



The factors which determine such a reversal of the polarity of the 

 egg may be different in different cases. Where the reversal is regular 

 and usual, as in sinistral gasteropods, it must be due to some peculiar 

 structure of the egg cell, possibly to a weakness of the protoplasmic 

 pellicle at the pole of attachment. Where inverse symmetry is very 

 unusual it may, perhaps, be due to pressure on the egg cell which 

 forces the spindle through the egg and causes the polar bodies to be 

 formed at the pole opposite to that at which they usually appear. 

 I have observed just such a reversal of the polarity of the egg of 

 Crepidula when under pressure, though I have not been able to follow 

 the later development of such eggs. What may, perhaps, be an 

 analogous reversal of polarity, though occurring after the maturation 

 and the fertilization of the egg and before the first cleavage, is found 

 in certain Ascidians (Ciona, Castle 1896) where the egg nucleus 

 moves away from the pole at which the polar bodies are formed and 

 after uniting with the sperm nucleus gives rise to the first cleavage 

 spindle near the opposite pole of the egg. 



Among all animals, with the exception of certain Ascidians, the 

 polar bodies are formed at the ectodermal or animal pole of the egg, 

 but, according to Castle, in Ciona, and probably in other Ascidians, 

 they are formed at the endodermal or vegetative pole, and this may 

 perhaps be due to a reversal of the polarity of the egg after the 

 maturation but before the first cleavage 1 ). Furthermore, if the view 

 which is here set forth is correct, it ought to be possible to produce 

 inverse embryos of the frog by inverting the egg immediately after it 

 is laid, in the manner described by Pflüger (1883), Born (1885), 



1) Since this paper was sent to press I have found that the polar 

 bodies are formed at the ectodermal pole of the egg in Ciona, Cynthia 

 and Molgula; these Ascidians, therefore, form no exception to the general 

 principle stated above. 



