Studies on Synapsis. 



71 



chromosomes are first recognised as attenuated filaments, progressively short- 

 ening into stout rod-like elements and undergoing cleavage simultaneously. 

 The same is true of the follicular nucleus : in prophase it is usually seen that 

 the separate chromosomes display appreciable differences of size inter se. 



A few equatorial sections of nuclei in metaphase, both from follicle cells 

 and oogonia, showed the chromosomes sufficiently widely separated to permit 

 an accurate count of the diploid complex. There are in such cases twenty- 

 four univalents : the unpaired element of the male germ cells is accompanied 

 by a homologous partner (fig. 17). Minute deeply staining granules are seen 

 among the chromosomes ; but it does not appear that their number is 

 constant, and there are suggestions that they are derived from the nucleolus 

 in the prophase (fig. 26). The evidence in Libellula depressa is against 

 the existence of a pair of ??i.-chromosomes like those described by Lefevre 

 and McGill in Anax. 



At the inception of the meiotic phase the chromatin organisation of the 

 nucleus consists of finely beaded, delicate, polarised loops in contact with 

 nuclear membrane along their entire length. This condition is of compara- 

 tively long duration in the case of the oocyte, since it is found with greater 

 frequency than other stages. Even in good sections it is difficult to count 

 the number of loops with great accuracy ; but by the study of sufficiently 

 thin sections (3 /j,) it is certain, that approximately, the full diploid number 

 of filaments is present. Thus it corresponds to the leptotene stage of Von 

 Winiwarter. It is illustrated in fig. 28. A plasmosome is always seen — 

 usually towards the polar side of the nucleus where the leptotene filaments 

 converge. There is no striking difference among the latter which would lead 

 to the identification of the accessories in the early meiotic stages. As in so 

 many other cases where the male X-element is conspicuously recognisable in 

 synapsis, the accessories of the female germ cells behave in all respects like 

 the autosomes. The only quahfication which this statement merits is that in 

 the metaphase of follicular, mitoses two chromosomes are distinctly larger 

 than their fellows. 



The bouquet figure now passes through the stages of contraction seen in 

 Periplaneta : the leptotene loops are withdrawn from the nuclear menrbrane, 

 become shorter and thicker, and are in consequence more closely approxi- 

 mated side by side. There is one noticeable difference between the two 

 cases ; in Libellula, as in Tomopteris, the process is not synchronous for all 

 the elements concerned ; hence, while it is difficult to state dogmatically that 

 there is a parallel conjugation in pairs of aU the loops, it is possible to affirm 

 without any shadow of doubt that such is the case with those which lag 

 behind the rest (fig. 29). 



