A Note on the Polarity of the Primary 

 Oocyte of Asterias forbcsii 



By 



Xaohide Yatsu. 



Zool. Inst., Tokyo Imp. Univ. 



While studying the artificial parthenogenesis of the egg of Asterias 

 forbesii at Woods Hole, I was struck by the fact that the axis of the 

 primary oocyte does not coincide with that of matured egg but the 

 axes make a certain angle to each other. So far this fact does not seem 

 to have been described, though perhaps it did not escape notice previously. 



Most eggs, as soon as shed into the water, assume a spherical or an 

 approximately spherical shape. The follicular membrane usually dis- 

 appears soon after a thin layer of gelatinous substance is formed {cf. 

 HARTMANN' '02 p. 802). " The nucleus [germinal vesicle] often holds 

 an eccentric position as regards the egg cell, though not infrequently 

 it is located centrally " (JORDAN" '08 p. 43). The germinal vesicle 

 begins to fade away on the part nearest to the surface of the egg, as has 

 been observed by MATHEWS, KING and others. And the polocytes are 

 formed there. In such eggs the spherical shape makes it impossible 

 to find any land-mark to determine the axial relation between 

 unmatured and matured eggs. 



Very seldom one comes across pyriform eggs with a thin follicular 

 membrane (Figs. 5-7). The membrane is usually drawn out a little 

 at the pointed end. Why they retain such a shape and their follicles 

 remain intact I do not know. Probably they are somewhat younger 

 than ordinary rounded eggs, though they go through the maturation 

 processes just as normally as the latter. At any rate the polocytes 



