102 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
unknown to occur in the history of any Phycomycetous sporangium 
or Floridean carpospore. Dangeard (’94), who first made its dis¬ 
covery, considers it a sexual act, but this conclusion scarcely seems 
plausible because the following facts appear to have been satis¬ 
factorily demonstrated in several instances: (1) there has already 
been a fusion of sexual elements in the ontogeny of the individual, 
(2) the fusing nuclei in the ascus are division products of nuclei 
belonging to the same cell, and perhaps in some cases even daugh¬ 
ters of sister nuclei, and (3) they are vegetatively active before 
fusion, as is the single nucleus after fusion. The phenomenon is 
probably vegetative rather than sexual, but its nature and signifi¬ 
cance will not be fully understood until further research reveals 
whether or not it is an acquired feature. 
Fortunately more exact comparisons can be instituted in dealing 
with the later events in the life of the ascus. Hence the homol¬ 
ogies of the ascus will at present be dealt with mainly from the 
standpoint of the phenomena of spore formation, and in the light of 
these phenomena each of the foregoing suggestions will be briefly 
considered in turn. 
Brefeld was the champion of the first suggestion for he regarded 
the sporangium of Mortierella or a closely related form as the 
ancestor of the ascus, but it is doubtful if his position is tenable on 
cytological grounds, though the cytology of this particular genus has 
not been described, yet owing to the careful work of Harper (’99) 
and recently of Swingle (:03) we know quite fully the cytological 
details of spore production in such closely allied genera as Pilobolus, 
Rhizopus, Sporodinia, and Phycornyces. In all of these forms, 
spore formation is initiated by a segregation of the spore plasm into 
a cell, the sporangium. The next step consists of a progressive 
cleavage of the spore plasm into irregular nmltinucleate masses, 
the cleavage beginning at the surface of a plasma or a vacuolar 
membrane. There is no residue of epiplasm. These masses may 
round themselves up into spores at once as in Sporodinia, or they 
may undergo further cleavage. In the latter case the later cleavage 
furrows do not curve, intersect, and branch in the same irregular 
way as do the earlier, but they seem to bear a more definite relation 
to the nuclei and in Pilobolus are first marked out by a development 
of hyaline zones. Harper is therefore apparently justified in con¬ 
cluding that since the method of sporogenesis in these sporangia dif- 
