Cytology of the A sous. 
543 
of chromosomes is apparent in the prophase as well as in the anaphase, the 
chromosomes having become paired at the beginning of the third division. 
In yet another species the pairing takes place in the second prophase, and 
when this happens, two divisions are concerned in brachymeiosis. In such 
cases the possibility is not excluded that the second reduction is accom- 
plished in the homotype, and that the chromosomes of the second telophase 
as well as those of the third are actually univalent. Nevertheless such 
a state of affairs appears to us improbable, since the ordinary role of the 
homotype is clearly to separate the products of a longitudinal fission 
occurring in the heterotype spireme. If, in certain cases, the second 
division is brachymeiotic, this separation must be delayed till the third 
division, and there seems no sufficient reason to suppose so radical a diver- 
gence in the reduction processes of closely related forms. 
A yet more essential difference between meiosis and brachymeiosis 
lies in the occurrence of a second contraction in meiotic reduction and its 
absence in the simpler process. If this phase be indeed the moment of 
interchange of material between the allelomorphs (Fraser and Welsford (9)) 
its absence in brachymeiosis may well indicate that this type of reduction 
and the corresponding asexual fusion have little effect on the forms in which 
they occur. Where the chromosomes are paired throughout the second 
division and in the prophases of the third, as in Ascobolus furfur aceus, 
another opportunity for the transfer of material may exist, but even this is 
absent in Lachnea stercorea and Humaria rutilans , where after meiosis the 
allelomorphs are at no time visibly associated. 
The fact that reduction in the number of chromosomes occurs after 
meiosis is complete has received confirmation not only from the work 
of Maire (13) and Guillermond (11), but more recently from Dangeard (4), 
who can scarcely be regarded as biassed in favour of a series of events 
which implies two fusions. He has recorded the numerical changes in 
question in Ascobolus furfur aceus, and is of opinion that they take 
place also in Pyronema confluens . In neither form, it must be added, has 
Dangeard found a sexual union in the ascogonium, but he has confined his 
investigations to the occurrence of a male organ, and has ignored the 
possibility of such a fusion in pairs of ascogonial nuclei as takes place 
in Humaria granulata. 
The evidence on this question has been discussed in earlier papers ; it 
seems unnecessary to enter again into an enumeration of the species in 
which two successive fusions have been seen. 
The Pairing of the Allelomorphs. 
Recent investigation has dealt extensively with the method in which 
the premeiotic chromosomes conjugate to form the gemini of the heterotype 
