CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 313 



Furthermore a general result of previous work has been to show 

 that the polarity and pattern of organization of an egg are not 

 changed by this dislocation of egg substances; the polarity and 

 pattern of the embryo which develops from such an egg remains 

 unchanged irrespective of the location of these different materials 

 within the egg. This is a surprising fact which in\dtes further 

 stud3^ How is it possible to dislocate in any axis the visible 

 material substances of an egg and yet leave its polarity and 

 pattern of organization undisturbed? 



Lillie ('06) concludes that polarity is a property of the 'ground 

 substance' of the egg, this substance being "a fluid which has no 

 filar, reticular or alveolar structure," but yet is 'firmly organ- 

 ized' so that it is not affected by centrifuging. He regards the 

 substances which are dislocated by centrifugal force as mere 

 'inclusions' in this 'ground substance,' consequently polarity 

 remains unchanged when these inclusions are forced to occupy 

 new positions since polarity inheres in the 'ground substance' 

 which is not moved by centrifugal force. It is evident from 

 Lillie's use of this term that he means the 'ground substance' to 

 include what is commonly called cytoplasm as contrasted with 

 metaplasm or inclusions. However it will be shown in this 

 paper that most of the cytoplasm of an egg can be displaced 

 without permanently changing the polarity of the egg. 



In eggs which contain relatively little yolk, such as those of 

 echinoderms, Chaetopterus, Cumingia, etc., the yolk may be 

 thrown to any pole without greatly displacing the protoplasm 

 from its normal position, and consequently it is possible in these 

 cases that normal development results because the real formative 

 materials, viz., nucleus and cytoplasm, have not been displaced 

 to any great extent by centrifugal force. But when the volume 

 of yolk is large, as in the egg of Crepidula, the nucleus and cyto- 

 plasm may be displaced from their normal positions by nearly 

 the whole diameter of the egg, and the subsequent developm nt 

 of such eggs throws light not only upon the specific value of dif- 

 ferent egg substances but also upon the polarity and organi- 

 zation of the egg as a whole. 



