Wilson.—Sex Determination in Mnium hornum . 435 
found in the younger archegonium represented in Fig. 1. In the organ 
shown in Fig. 2 two naked cells are present in the venter, and the canal only 
contains a small quantity of mucilaginous material. In this case it appears 
probable that the contents of the neck have been ejected, leaving the ovum 
and ventral canal-cell in the cavity of the venter ; unfortunately the upper 
part of this organ had been destroyed during manipulation. Although it is 
doubtful whether fertilization could have taken place in the structures repre¬ 
sented in Figs. 1 and 3, there appears to be no reason why one of the cells 
present in the venter of the organ shown in Fig. 2 should not have functioned 
as an ovum. In Fig. 4 three cells are apparently present in the venter. 
The organ shown in Fig. 5 has the general form and appearance of an 
antheridium, but differs in being longer and narrower than the normal 
organ. The upper part contains numerous spermatids, while in the central 
part of the lower portion two cells are present which bear a close resem¬ 
blance to an ovum and ventral canal-cell, both in appearance and position. 
The spermatids closely resemble those found in the normal antheridia, and 
little or no production of mucilaginous material appears to have taken 
place. The organs previously described may perhaps be looked upon 
as modified archegonia, but that shown in Fig. 5 is obviously bisexual. 
It has been already pointed out that El. and 6m. Marchal have 
discovered organs of mixed sex in aposporously produced plants of Mnium 
hornum ) and it might be urged that the organs just described were borne on 
an individual which had been aposporously produced. It has been shown 
by Brizi ( 3 ) that capsules of Funaria hygrometrica can give rise to protone- 
mata while still attached to the living moss plant, and it is not improbable 
that apospory may sometimes take place in nature. 6l. and Em. Marchal (12) 
have shown that a plant produced in this way would possess the diploid 
number of chromosomes. An examination of the normal antheridia was 
therefore made, and this led to the discovery of one in which divisions of the 
spermatogenic cells were taking place. The dividing cells are in the 
condition of late prophase, and there is little difficulty in determining the 
number of chromosomes (Figs. 6 , 7, and 8). A number of counts were made, 
and in all cases six chromosomes were present. As it has been previously 
shown ( 18 , 19 ) that this is the haploid number for Mnium hornum , it 
is evident that the plant in question did not have an aposporous origin. 
In view of the discovery of organs of both sexes on a single axis 
of Mnium hornum , a further examination of the results obtained by El. and 
Em. Marchal is rendered necessary. According to these investigators there 
is an absolute separation of the sexes at the reduction division. The 
unisexual character is retained throughout the haploid phase, and the 
reunion of the sex determinants is brought about by fertilization. ‘ La 
reduction chromatique . . . est, a coup sur, la cause determinante de la 
disjonction sexuelle. Le caractere unisexue de la spore conserve rigou- 
