585 



that the ovogonia and the early stages of the ovocyte are diplopolar 

 and no doubt the same is true of the free ovocyte. 



If the nucleus and the structures at the free pole were carried 

 through the egg to the opposite pole while at the same time all parts in 

 any transverse plane of the egg preserve their same relative positions 

 in that plane we should have a total inversion of symmetry, Fig. 1. 

 That this actually happens in the case of sinistral gasteropods seems 

 to me extremely probable though as yet I have been unable to demon- 

 strate it. In my work on Crepidula (1898, 1902) I found that there 

 were slow movements of the egg substance through the egg axis from 

 the vegetative to the animal pole. As a result of these movements 

 the germinal vesicle, or the mitotic figure to which it gives rise, 

 together with the surrounding cytoplasm move to one pole of the egg 

 and here the polar bodies are formed. Similar movements may be 

 seen to occur in the living eggs of Physa, Planorbis and Limnaea, 

 though owing to the lack of landmarks it is difficult to prove that 

 these movements are toward one pole in dextral forms and toward 

 the opposite pole in sinistral ones. However in the case of both 

 Physa and Planorbis the well marked polar differentiation of the 

 attached ovarian egg is lost or obscured after the egg is set free; 

 the eccentric position of the nucleus gives place to a central one and 

 the aggregation of cytoplasm at the free pole and of yolk at the 

 pole of attachment is followed by a condition in which the yolk is 

 pretty uniformly distributed around the periphery of the egg. This 

 is just what would be expected if a reversal of the polarity of the 

 egg occurs at this stage in these sinistral snails. 



Unfortunately it is not only very difficult to get eggs in these stages 

 which have to be dissected out of the hermaphrodite duct and uterus, 

 but also difficult to keep them living and in a normal condition while 

 watching these movements. For these reasons I have been unable 

 as yet to observe the actual movements of the egg substance at this 

 stage. It was not until the close of the present breeding season of 

 these snails that I became aware of the exact time and place at 

 which this reversal of polarity must occur, if it occurs at all; for 

 more than a month past I have been unable to obtain actively laying 

 snails, and the continuance of this investigation must be postponed 

 until next spring. For this reason I have thought it advisable to 

 to publish these results as they stand, although I am aware that 

 they fall short of actual proof. Nothing less than the observation 

 of the movement of the nucleus and cytoplasm through the living egg 



