Cytolological Problems. 231 
recent years, the divergences have become more pronounced. 
Unfortunately the exchange of preparations initiated by some 
animal histologists has done nothing to bridge the gulf between 
divergent views; different observers draw different conclusions, even 
from the same preparations. This suggests that in many of these 
cases the phenomena observed are not really sufficient to enable 
one to come to an accurate conclusion as to the behaviour of the 
chromosomes. The two points that are really in dispute are, first, 
whether a parallel conjugation of chromosomes takes place in the 
prophase of the reduction-division, and secondly, how this association, 
if it occurs, is brought about. In the unsatisfactory state of the 
evidence to be obtained from direct observations Strasburger 
considers that general considerations ought to be brought to bear to 
help to elucidate the problem. 
If there is no pairing of chromosomes and the “ gemini ” are 
merely the result of a longitudinal splitting of an original single 
thread, the production of the special meiotic apparatus of the 
tetrad division seems quite superfluous, for that division becomes 
simply two successive ordinary divisions. Again the two halves of 
the gemini behave quite differently from the two halves of an 
ordinary longitudinal chromosome as seen in a somatic division ; in 
the second case the two halves remain close together and only 
separate in the metaphase, while the halves of the “gemini” become 
widely separated often throughout nearly their whole length. 
Further the close relation, often overlapping, of the two divisions 
remains incomprehensible if both are merely ordinary divisions. 
If, also, there is no conjugation of chromosomes it is not at all clear 
why the number of elements is halved and it is only to be explained 
by secondary hypotheses; whereas a pairing of the elements 
gives an immediate explanation of the reduction. 
There is also the further question as to how the pairing of 
chromosomes is brought about, whether by parallel conjugation in 
the early prophase as Gregoire and Strasburger hold, or by lateral 
approximation in the late prophase of segments of a double 
chromosome formed by the union of ordinary chromosomes end to 
end, as held by Farmer and Moore and by Mottier. Strasburger 
supports his view on the grounds of the pairing of homologous 
somatic chromosomes which have been observed in many cases and 
are especially clear where the chromosomes are unequal in size as in 
Galtonia and Funkia. Also on the ground that if the gemini are 
produced by the folding together of chromosome segments the 
synapsis stages and the curious stretching out of the chromosomes 
into long threads in the early stages of prophase remain without 
satisfactory explanation. The simple pairing of homologous 
chromosomes requires no such elaborate arrangement, for it is found 
in the somatic nuclei of the plants just mentioned. Strasburger 
sees in the lengthening out of the chromosomes a means of 
separating the pangens and allowing the opposition of homologous 
pairs in homologous chromosomes. 
Strasburger professes his scepticism as to the occurrence in 
Polytrichum of two reductions and a peculiar fertilisation by two 
spermatazoids, as described recently by two Dutch botanists. He 
also, though it seems with much less reason, throws doubt upon the 
double nuclear fusion and double reduction in the Ascomycetes. 
