The spermatogenesis of Peripatus (Peripatopsis) balfouri. 293 



This form of metakinesis is interesting as it is in a way inter- 

 mediate between that of general mitoses and that of heterotypic 

 mitoses : were the daughter chromosomes to remain connected for a 

 time by both ends instead of by one end, we would have the form of 

 metakinesis found in heterotypic metakinesis. On the completion of 

 metakinesis the daughter chromosomes are pulled to opposite poles of 

 the spindle; in each chromosome one pole is directed towards the 

 spindle, the other directed away from it (Figs. 43 — 48). Corresponding 

 daughter chromosomes remain connected by connective fibres, which 

 are much thicker than the mantle fibres. In each plate of chromo- 

 somes (Figs. 53 — 55, 37 B) their number is exactly the same as the 

 number in the monaster stage, namely 28 : the metakinesis halves each 

 and every chromosome. At first the long axes of the chromosomes 

 are more or less parallel to the long axis of the now elongate spindle ; 

 subsequently the ends nearest the centrosomes become pulled inwards 

 so that their axes make all angles with that of the spindle. The 

 centrosomes are exceedingly minute, but in a few cells of this dy aster 

 stage I could find that each had divided into two; from this stage 

 through the rest stage of the spermatocytes I could not determine 

 the presence of centrosomes with any certainty. 



IV. Anaphases of the Spermatogonic Mitoses. 



These may for the sake of convenience be divided into early 

 ■anaphases and synapsis stages. 



1. Early Anaphases. 



In the dyaster stage the achromatic spindle has elongated so that 

 its poles come nearly or quite into contact with the cell membrane, 

 and the cell body itself becomes stretched out in the same line 

 (Figs. 48, 57); since pole radiations cannot be seen, this process can 

 be well ascribed to the agency of the elastic straightening of central 

 spindle fibres (DrCner). It is to the usually assumed agency of the 

 contraction of the mantle fibres that we must ascribe the movement 

 of the chromosomes to the poles. As we have seen, the metakinesis 

 was of such a kind that one end of each daughter chromosome comes 

 to lie nearer the pole of the spindle than does the other end. As 

 we shall see that the two ends of each chromosome can be followed 

 with perfect certainty in all the later stages up to the spermatid, it 

 will be found convenient to specify each end of the daughter chromo- 



