6 INVEETEBRATA chap. 



This is the aggregation of the chromosomes in a kind of bunch at 

 one side of the nucleus. This persists for a time, but eventually 

 the chromosomes separate out again from one another, a process 

 known as diakinesis. The rings of the strepsitene stage are com- 

 pletely broken up into their constituent parts, and the number 2 x 

 is consequently restored by the time that diakinesis has reached its 

 utmost limit. 



During diakinesis the chromosomes become driven apart from 

 one another and become spread out under the nuclear membrane, 

 and the nucleolus during the same time gradually loses its staining 

 power and disappears from view. The nuclear membrane then 

 disappears and the nuclear sap mingles with the cytoplasm (Fig. 1, D). 

 At the same time, the centrosome, which forms the centre of the polar 

 rays in every nucleus undergoing karyokinesis, and which lies at the 

 side of the resting nucleus, is seen to have divided into two daughter 

 centrosomes (Fig. 1, E c) ; they are already moving apart to take up 

 positions at opposite poles of the nucleus, whilst between them the 

 rays of the achromatic spindle are already developing. 



The chromosomes, which become much shorter and thicker, 

 now join end to end in pairs ; but their free ends then swing round 

 and join one another, so that eventually parasyndesis again occurs 

 (Fig. 1, F &\ W, and W), and the number of chromosomes is again halved, 

 and we thus obtain x short, thick, ring-like chromosomes which 

 become arranged in a plane so as to form the equatorial plate of the 

 first spindle. 



The division of the nucleus then takes place ; each daughter 

 nucleus receives one half of each ring, and these halves represent the 

 separate chromosomes of the diaJdnetic stage which subsequently fused 

 with another (Fig. 1, G). 



In the case of the testis the division of the nucleus is followed by 

 the division of the germ cell into two equal cells, and in this way two 

 spermatocytes of the second order are formed : the undivided germ 

 cell with its nucleus and condensed chromosomes being termed the 

 spermatocyte of the first order. The nuclei of the spermatocytes 

 of the fir^ order do not undergo a resting stage, but in each of 

 them a new spindle is formed at right angles to the first, and each 

 chromosome becomes split longitudinally so that at the ensuing 

 division of the nucleus one half of each chromosome goes into each 

 daughter nucleus (Fig. 1, H). 



If this description has been followed it will be seen that, in the 

 first maturation division whole chromosomes go to each daughter 

 nucleus — this is termed the reducing division ; whilst in the second 

 division one half of each chromosome goes to each daughter nucleus 

 — this is termed the equating division. 



The division of the nucleus is again followed by the division of 

 the cell, and in this way four cells are derived from each spermato- 

 cyte of the first order. These cells are termed spermatids and each 

 spermatid becomes converted into a spermatozoon. The manner in 



