80 MATURATION OF THE EGG [CH. 



ever, are evidently superficial, depending on the nature 

 and function of the cells concerned; fundamentally the 

 process is similar in the two sexes. This similarity is further 

 illustrated by the observation of FRANCOTTE that in the 

 Polyclad Prosiheceraeus the first polar body is nearly as 

 large as the egg-cell. It undergoes an unequal division, 

 giving off a small second polar body, just as the egg-cell 

 does, and may be fertilised by a spermatozoon and segment 

 as far as the gastrula stage. This production of a giant 

 polar body has been imitated by CONKLIN (1917) in the eggs 

 of the Mollusc Crepidula by centrifuging them during the 

 maturation divisions. His results lead him to formulate an 

 interesting hypothesis to account for the rather puzzling 

 fact that in the production of normal polar bodies the cell 

 divides extremely unequally in a way that contradicts the 

 rule of protoplasmic mass referred to in Chapter III. He 

 finds that if an egg is centrifuged violently during the pro- 

 phase of the polar division, the whole polar spindle may be 

 carried from the egg surface into the centre. If, however, 

 the mitosis has reached the metaphase or anaphase stage, 

 it is impossible to draw the outer pole of the spindle from 

 the surface, with the result that the whole spindle becomes 

 greatly elongated, and when the cell divides, the polar body 

 separated off is relatively enormous. He concludes, there- 

 fore, that the polar spindle is firmly anchored, especially in 

 the later stages, to the elastic " spongioplasm " of the egg 

 surface by means of its outer pole or aster, while the inner 

 pole is relatively free. The inequality of the polar divisions 

 thus arises from the firm attachment of the mitotic spindle 

 to the egg surface by one pole only, while in ordinary 

 divisions, such as of a segmenting egg or of a spermatocyte, 

 the poles are usually symmetrical in this respect. In this 

 connection, and as showing still further the homology of 



