Division in Mnium hornum. 
*53 
arrangement of the spireme can be seen after emergence, and close approxi- 
mation of the two threads goes on before the second contraction, resulting 
in the formation of the thick spireme. Later on this gives rise to the 
chromosomes by transverse divisions. The latter consist of two somatic 
chromosomes in close lateral approximation. At the metaphase the 
approximated somatic chromosomes are separated. The difference 
between the two theories lies essentially in the formation of the split 
spireme ; in the first this is considered to be due to longitudinal fission 
of an original single thread, in the second to the lateral approximation 
of originally separate filaments. 
In the case of Mnium no evidence for lateral approximation was 
discovered during or previous to the first contraction. Contraction goes 
on in the network as a whole, and there is no rearrangement of the thread 
into parallel lengths. Sections of the contracted mass likewise gave no 
evidence for approximation. No signs of longitudinal fission were found 
in the emerging thread, but traces of this were discovered at the time of 
second contraction, and also in the differentiating chromosomes, and no 
doubt is entertained as to its presence at this stage. The small size of the 
nuclei in the plant under examination makes satisfactory elucidation of the 
events of chromosome formation difficult, but the evidence obtained con- 
firms the conclusions arrived at by Farmer and Moore. 
The above investigation into the nuclear phenomena of Mnium hornum 
has proved that both in the somatic and in the reduction divisions the 
normal course of events is followed ; the first division of the spore-mother- 
cells is of the characteristic type. The chromosome numbers found in the 
Muscineae up to the present are comparatively small. The only recorded 
number for the diploid generation is twelve, and in the haploid genera- 
tion eight, given by Ikeno for Atrichum angustatum and Pogonatum 
rhopolophorum , is the only variant from the six found in Mniinn hornum and 
various species of Poly trichum. It may be taken that twelve is the 
characteristic number for several individuals of the Muscineae. The latter 
agree with the Hepaticae in the possession of a comparatively small 
number of chromosomes. 
The absence of centrospheres during the reduction division is a note- 
worthy fact. These structures have been described in a considerable 
number of representatives of the Hepaticae during the meiosis, and their 
absence in Mnium may perhaps be correlated with the greater specializa- 
tion for the land habitat found in the Muscineae. 
In their examination of Poly trichum , Drs. Leeuwen-Reijnvaan were 
unable to obtain the reduction division, but since twelve chromosomes 
were found during vegetative divisions in the sporogonium, and six during 
division in the antheridiurn, they concluded that reduction took place as 
usual on spore formation. This, which was already justified by analogy 
