Studies on Synapsis. 



69 



relatively of enormous size ; and it exhibits usually a double structure, having 

 a central large vacuolar area ; in addition, there is usually an extranucleolar 

 Dody lying free in the nucleus. With a view to obtaining further data 

 respecting the behaviour of the plasmosome in oogenesis, and with a desire to 

 test the validity of McGill's interpretation of the similar double nucleolus 

 of Anax and Plathemis,* the present research was begun ; in the course of 

 the work other points of interest emerged, as described above. 



The point of signal interest in McGill's paper is the contention that the 

 double nucleolus of the growing oocyte is formed by the condensation of the 

 synaptic spireme (i.e., bouquet) round the plasmosome. This process is 

 homologised with synizesis in the male germ cells. The definitive chromatin 

 organisation of the later egg is supposed by McGill to be formed by precipi- 

 tation of substance secreted as globules by the nucleolus and dissolved in the 

 karyolymph. The whole series of phenomena, it is maintained, reinforces the 

 now generally discarded belief of Eetzius, Leydig, Strassburger, and others, 

 who regarded the plasmosome as a means of storing chromatin. 



This is clearly at variance with (1) the general hypothesis of chromosome 

 individuality, and (2) most accredited data bearing on the meiotic phenomena 

 of oogenesis. It is usual to find in oogenesis, as in the case of Periplaneta, or 

 Gryllus, that the diplotone filaments or pro-tetrads tend to become 

 increasingly granular as the oocyte enters upon the phase of active growth in 

 cytoplasmic bulk, so that typically the whole surface of the oocyte nucleus 

 appears to become covered with uniformly distributed microsomes (chromatin 

 granules) during the major part of the history of the egg. Eventually, the 

 onset of the reverse process occurs, when the polar spindle is formed, i.e., 

 about the time the egg is laid or fertilisation occurs. In insects, with the 

 exception pre-eminently of Symenopter a parasitica (Hegner, Gatenby, Hogben), 

 maturation does not begin in the ovarian egg. In a few cases, as in certain 

 Amphibia, there is no " difi'use stage " at all (Jordan, King) ; the bivalents 

 persist without such change throughout the entire growth of the oocyte. In 

 yet other cases the bivalents become very diffuse, but are still individually 

 recognisable, as in Pristiurus. 



Prom these circumstances, and the fact that the chromatin units always 

 reappear in the prophase, in the same manner and the same form as that in 

 which they pass into the reticulate condition, it seems legitimate to infer, in 

 the absence of evidence to the contrary, that there is no actual loss of 

 individuality in the chromatin organisation of the oocyte. Evidence to the 

 contrary has indeed been submitted and criticised in a previous contribution 

 to this series, and further data bearing on this question will be considered 

 * The genus Plathemis has now been merged in Libellula. 



