II SYNIZESIS 55 



A comparison of the figures of Lepidosiren and Oncopeltus shows that 

 the intensity of the contraction varies. Oncopeltus forms an intermediate 

 stage between the extremely dense contraction in Lepidosiren and its 

 complete absence in Tomopteris. 



The exact moment at which synizesis begins and ends also varies 

 considerably. In Lepidosiren it begins with the onset of the diplotene 

 stage. In Oncopeltus it apparently corresponds with the zygotene stage 

 — at any rate it occurs between the leptotene and pachytene stages. 

 More frequently, perhaps, it sets in earlier still, when the leptotene threads 

 are emerging from the resting nucleus. 



Finally, in many forms the telophase contraction of the last sperma- 

 togonial mitosis has been described as passing directly into synizesis 

 ^vithout an intermediate diffuse stage. As this conclusion is based upon 

 the negative evidence of failing to find diffused stages between the two, 

 these accounts should perhaps be accepted with some reserve. On the 

 other hand, there seems to be no theoretical reason to doubt them, and 

 so many different observers have given such accounts that it is difficult 

 to doubt their combined testimony. As examples may be quoted 

 Peripatus (Montgomery, 1901 a), and Scohpendra (Blackman, 1905). 



A few cytologists have described two synizetic contractions, separated 

 by a diffused stage. This observation has been upheld especially by 

 Farmer and Moore (1905) and by the adherents to their telosyndetic 

 scheme, in which the second contraction is supposed to bring about the 

 doubhng over of the pachytene bands to form the rings, etc., of the 

 bivalents. 



C. MEIOSIS IN THE FEMALE 



So far we have confined our description of meiosis to that process as it 

 occurs in the male. The behaviour of the chromatin in oogenesis is 

 closely parallel to that in spermatogenesis, with, however, modifications 

 connected with the long-growth period of the oocyte and the compara- 

 tively gigantic size of the mature egg. The deposition of the yolk and 

 the growth of the oocyte I. take place between syndesis and metaphase I. 

 During this period, which may endure for months or years (mammals), 

 the chromosomes may not retain their chromosomal forms, but often 

 nearly, or quite, vanish into a peculiar form of resting nucleus known 

 as the germinal vesicle, to reappear immediately before the first meiotic 

 division. This special feature of oogenesis does not, however, destroy 

 the fundamental similarity between male and female meioses, and the 

 above discussions regarding syndesis, pre- and post-reduction, synizesis, 

 etc., apply as well to oogenesis as to spermatogenesis. Even the germinal 

 vesicle stage, as we shall sec, is paralleled in the spermatogenesis of 

 certain animals. 



