18 



BULLETIN : MUSEra OF COMPARATIVE ZOOLOGY. 



blastotnero A, while a^-* represents the most dorsal one, the others 

 occupyiug iutermediate positions. 



First Cleavage. 



After the formation of the polar cell the nucleus (Plate 1, Fig. 2) 

 returns to the position formerly occupied by the germinal vesicle in the 

 longitudinal axis of the egg, lying nearer that end of the ovum in prox- 

 imity to which the polar cell was formed (the raicromere end). It takes 

 such a position that a plane at right angles to the long axis of the egg 

 and cutting the polar cell would also cut the nucleus. The distance 

 from the centre of the nucleus to the nearer end of the egg is about 

 two fifths of the length of the egg. Here two asters appear on oppo- 

 site sides of the nucleus, the line joining them being oblique to the 

 long axis of the egg. Though I have examined a large number of eggs 

 at this staire, in no case have I been able to observe a stage in the 

 process of forming the two asters of the first cleavage spindle (Fig. 3) 

 from the single aster remaining after the formation of the polar cell 

 (Fig. 2). 



Between the two asters a spindle is formed. This lies at first some- 

 what oblique to the longitudinal axis of the egg, as shown in Figure 3, 

 but before cleavage takes place the spindle swings into coincidence with 

 the long axis (Fig. 4). The aster lying at the raicromere end of the 

 spindle is distinctly smaller than the opposite one (Fig. 3). The nucleus 

 becomes divided into two small masses, which move toward opposite 



