476 ROBERT W. HEGNER 



Here, however, the pole-disc granules were considered the es- 

 sential substance. 



The appearance of the Keimbahn-determinants at a certain 

 time and in a certain place, and their determinate segregation 

 point unmistakably to an underlying regulating mechanism. 

 These phenomena have some definite relation to the fundamental 

 organization of the egg and require an investigation of our pres- 

 ent knowledge of this subject. 



The isotropism of the egg, as postulated by Pfliiger, and the 

 'cell interaction' idea, especially developed by O. Hertwig and 

 Driesch, have given way before the beautiful researches tending 

 to uphold the hypothesis of 'germinal localization' proposed by His 

 and championed by so many investigators within the past two 

 decades. The starting point for embryological studies has 

 shifted from the germ layers to the cleavage cells and from these 

 to the undivided egg. Organization, which Whitman ('93) 

 maintains precedes cell-formation and regulates it, is now traced 

 back to very early stages in the germ cell cycle and is held re- 

 sponsible for the cytoplasmic localization in the egg. 



One of the fundamental characteristics of the egg is its polarity. 

 It has been known for about thirty years that the eggs of insects 

 are definitely oriented within the ovaries of the adults. Hallez 

 ■ in 1886, finding this to be true of the ova of Hydrophilus and 

 Locusta, expressed the fact in his "Loi de I'orientation de I'em- 

 bryon chez les insectes" as follows: "La cellule-oeuf possede la 

 meme orientation que I'organisme maternal qui I'a produit: elle 

 a un pole cephalique et un pole caudal, un cote droit et un cote 

 gauche, une face dorsale et une face ventrale; et ces differentes 

 faces de la cellule-oeuf coincident aux faces correspondentes de 

 I'embryon." Moreover, gravity and the action of centrifugal 

 force have no effect upon polarity of insect eggs (Hegner, '09b). 

 Giardina ('01) has found that during the divisions of the oogonia 

 in Dytiscus, a rosette of sixteen cells is produced, one of which 

 is the oocyte and the other fifteen nurse cells. The rosette thus 

 formed possesses a definite polarity coincident with the axis of the 

 oocyte which is identical with that which was present in the 

 last generation of oogonia. Similarly in Miastor (fig. 27) the 



