72 



Mr. L. Hogben. 



In such cases it can be seen, as might be anticipated from the orientation 

 of the loops, that approximation begins at the polarised extremities of the 

 filaments extending distally till there is only a small portion where they still 

 remain apart. The thickening of the filaments and their reduction 

 numerically, as will be seen later, is effected in this process, and a considera- 

 tion of the disposition of the loops makes it evident that reduction could only 

 be brought about by parasyndesis. Though the loops, now bivalent, contract 

 considerably towards the polar side of the nucleus with plasmosome to the 

 side of or in the centre of the bouquet figure (fig, 30), in really good prepara- 

 tions there is nothing strictly comparable to the compact synizesis of some 

 authors. A comparison between what occurs in Libellula and McGill's 

 figures of Anax and Plathemis will suffice to make clear that what is 

 described as the condensation of the synaptic spireme round the nucleolus in 

 these two genera corresponds to maximal contraction in Libellula (fig. 30). 

 Indeed, precisely similar appearances to those described by McGill are seen 

 in preparations of Libellula made originally by the technical methods 

 employed by that investigator. 



The chief hiatus in McGill's account of the meiotic phase of the oocyte in 

 Anax and Plathemis concerns the subsequent events. And there can now be 

 little doubt that this is due to the short duration of the post-synaptic stage 

 which intervene before the diffuse condition of the oocyte nucleus. It is a 

 curious fact that, whereas in probably the majority of animals, the diplotene 

 stage is that of longest and leptotene of shortest duration in the series of meiotic 

 phenomena, the immediately post-synaptic stages in the oocytes of Libellula 

 are extremely rare compared with the earlier ones. Thus a Liberal supply of 

 nymphs must be available if the complete history is required. And here 

 opportunity arises to reiterate the paramount necessity of examining very 

 large quantities of material for elucidating the whole sequence of events in 

 the nuclear history of the germ cells : probably there is no more common 

 occasion for error that has arisen in connection with the work which has been 

 published in relation to the controversies which centre round the interpreta- 

 tion of synapsis than the extreme rapidity with which in certain cases some 

 of the most critical stages are passed. For this reason negative evidence 

 such as the failure of Nakahara and Arnold to find typical leptotene stages 

 cannot carry the conviction which its exponents appear to anticipate. 



After remaining for some time in maximal contraction, the pachytene 

 loops become sufficiently loosened to permit exact numerical estimation 

 (fig. 31) in sections at right angles to the axis of the bouquet: it is then 

 seen that only the haploid number are now present. Numerical reduction 

 is thus effected with certainty in the contraction process. As the pachytene 



