No. 2.] ORGANIZATION OF THE EGG OF UNIO. 257 



phenomena repeatedly with the utmost care, and have demon- 

 strated the sections to several colleagues with perfect satisfac- 

 tion. The inner sphere at the central end of the spindle 

 becomes the sphere substance of the fertilized &%%. The evi- 

 dence that the sphere substance formed at the poles of the 

 first cleavage spindle has a similar centrosomic origin is 

 equally conclusive, as the figures show. It seems very prob- 

 able, although I cannot speak with assurance on this point, 

 that part of the material for the growth of the sphere sub- 

 stance is derived from the nucleus. If this be so, the later 

 wide dispersal of this substance through the cell may give a 

 useful clue to the manner in which the nucleus exercises its 

 undoubted formative influence on the cell. 



A similar growth of the spheres towards the end of karyo- 

 kinesis is now known to be of very wide occurrence (Agassiz 

 and Whitman, '89; Boveri, '95; Erlanger, '98; Herfort, '99; 

 Conklin, '99; Vejdovsky, '88). Opinions as to the morpho- 

 logical significance of these structures vary. Erlanger ('98) 

 declares: " Ferner entsprechen die Centroplasmen oder ' Spha- 

 ren ' durchaus nicht riesig angeschwollenen Centralkorpern wie 

 Boveri angiebt." My own observations would support Boveri's 

 interpretation. Erlanger also suggests an interaction between 

 nucleus and sphere substance : " Den hier entwickelten An- 

 schauungen gemass diirfte in den mittleren Phasen der Mitose 

 ein enges Verhaltniss zwischen den Centroplasmen, inclusive 

 Centralkorpern, und dem Kern, beziehungsweise Kernspindel, 

 herrschen." Herfort's observations ('99) lend themselves to a 

 similar interpretation. 



Conklin ('98) was the first to recognize the possible impor- 

 tance of the sphere substance in differentiation. He summa- 

 rizes his results thus : In the (tgg of Crepidula, " after the first 

 two cleavages, the sphere substance is differently distributed to 

 the different cells, the entire sphere substance of one genera- 

 tion always going into those cells of the next generation which 

 lie nearest the animal pole. This differential distribution of 

 the spheres has been followed through every cleavage up to 

 the 24-cell stage. As the form of cleavage is perfectly con- 

 stant, it follows that the sphere substance of any generation 



