324 WILSON AND MATHEWS. [Vol. X. 



definitive polarity is not primordial, but is induced by causes 

 operating at the time the segmentation-nucleus takes up its 

 position, then it is brought into the category of epigenetic 

 phenomena, 



2. Fertilization and Cleavage in the living Egg. 



When the pronuclei unite (7-8 m.) the rays of the sperm- 

 aster extend far out into the vitellus, often nearly to the 

 periphery. The central body of the aster then becomes much 

 enlarged ("aureole phase" of Fol) and divides into two 

 halves, which diverge to opposite poles of the segmentation- 

 nucleus (12-16 m.). The latter meanwhile slowly proceeds to 

 its definitive eccentric position. A long pause now ensues, 

 during which the astral rays become less marked, and finally 

 almost disappear, while the nucleus considerably increases in 

 size (20-30 m.). At 25-30 m. the nuclear membrane fades 

 away quite suddenly, disappearing first at the poles nearest the 

 archoplasm-spheres, and wholly fading away within two or 

 three minutes. Astral rays now again extend themselves 

 from the archoplasm-spheres, a beautiful amphiaster is devel- 

 oped (3 5-40 m.), and the first cleavage takes place (45-50 m.). 



The axis of the cleavage-spindle is always nearly at right 

 angles to the copulation-path ; and since the angle between 

 copulation-path and entrance-path is small the plane of first 

 cleavage is, in the great majority of cases at least, approxi- 

 mately through the entrance-point of the sperm (Fig. i).^ 



B. Fertilization and Cleavage in Sections. Toxopneustes. 



I, Structure of the first Cleavage-amphiaster. 



The history of the archoplasmic structures may best be con- 

 sidered by beginning with the first cleavage-amphiaster at a 

 stage corresponding with Fol's Fig. 10. In general appear- 



1 This was determined by the continuous observation of about seventy cases. 

 In only one case was a deviation as great as 45° observed ; it was rarely as great 

 as 15°. This agreement proves that no serious error can have been committed 

 regarding the axial relations, described above, through accidental rolling of the 

 eggs. 



