ZYGOMYCETES 109 



the gametangium, the sporiferous hypha is the suspensor and the whole 

 outgrowth is called copulation branch. As the homothallic forms are 

 bisexual, there apparently takes place in their hyphae, at the formation 

 of the copulation branches, a spatial separation of + and — energids. 

 In some forms, the copulation branches may arise from ordinary hyphae; 

 in others they are formed on special branches, the zygophores. 



The separating double wall between the gametangia is gradually 

 dissolved from the middle toward the edge and the zygote becomes a 

 hypnospore by the formation of a many-layered wall, the zygospore 

 (Fig. 66, 4). If in the homothallic forms, the copulation branch finds 

 no mate, in many species the gametangium is surrounded by a many- 

 layered wall, and is called an azygospore (Fig. 66, 8 and 9) or chlamydo- 

 spore. The same thing occurs if the cultures are removed to unfavorable 

 conditions, such as high temperatures. In the heterothallic forms 

 similar phenomena may occur if the copulation branches belong to 

 two different species; in this case they cease growing and transform the 

 gametangia (in case they have already been cut off as such) into azygo- 

 spores. This incomplete hybridization, however, does not seem to occur 

 between all species, for it occurs between Phycomyces nitens + and 

 Mucor Mucedo — and conversely, but not between Phycomyces nitens 

 and Rhizopus nigricans (Blakeslee, 1904, 1915, 1921, 1927). 



While both gametangia are usually of the same size and thus extern- 

 ally suggest isogamy, in individual species their size relationships show a 

 notable tendency to heterogamy. Thus in the homothallic Zijgorhynchus 

 Moelleri, the copulation branches are unequally developed. In the 

 heterothallic Absidia Orchidis, the gametangia are unequally broad, so 

 that the zygospore is conical. In Piptocephalis, the zygospores grow 

 upward from the point of fusion so that it is borne upon the top of the 

 copulation branch (Fig. 63). In Syncephalis nodosa, one copulation 

 branch coils around the other in a helix (Thaxter, 1897) ; the zygospore 

 does not arise at the point of fusion but comparatively distant, on the 

 outer wall of the helix near the septum separating the gametangium from 

 the suspensor. Still more puzzling is Dispira americana (Thaxter, 1895a), 

 parasitic on other Mucoraceae, where, at the tips of the vegetative hyphae, 

 thicker fertile branches swell and approach the sporangiophores of the 

 host, forming small sinkers. A septum divides the swollen part into two 

 cells, both gametangia. After copulation the proximal cell swells and 

 changes to a zygospore which is surrounded with digitate processes of the 

 distal cell. 



All these phenomena may be referred to a unified basic form as soon 

 as one considers the cytological relationships. As an example may serve 

 the homothallic Sporodinia grandis (Leger, 1895; Dangeard, 1906; 

 Lendner, 1908;Moreau, 1913;Keene, 1914). Its young gametangia contain 

 more than a thousand nuclei (Fig. 67, 1). While the separating wall 



