MUCORALES 



107 



copulating branches belong to two different species; in this case, they cease 

 growing and transform the; ganietniigia (in case they liave ali'(!ady been cut 

 off as such) into azygospores. This inconii)lete hybridization, however, does 

 not seem to occur between all species, for, while it occurs between Phycoriiycea 

 nitens + and Mucor Mucedo - and conversely, it does not between Phycomyces 

 nitens and Rhizopus nigricans (Blakeslee 1904-1927), 



While both gametangia are usually of the same size and thus externally 

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

 tendency to heterogamy. Thus, in the homothallic ZygorJiyncJins Moelleri, 

 the copulating branches are unequally developed. In the heterothallic Absidia 

 Orchidis the gametangia are of unequal diameter, the resulting zygospores being 

 conic. In Piptocephalis, the zygospore grows upward from the point of fusion 

 so that it is borne upon the top of the copulation branch. In Syncephalis 



Fig. 10. — Development of Uie zygospore of Sporodinia yrandis. (After Keene 1914.) 



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 portion of the helix near the septum separating the gametangium 

 from the suspensor. 



Cytologically all the; above-mentioned processes behave similarly. Sporo- 

 dinia grandis has been more complete studied. Its young gametangia con- 

 tain more than a thousand nuclei each (Fig. 10). While the separating walls 

 between the gametangia are dissolved, the nuclei undergo almost simultaneous 

 division ; their cytoplasm intermingles and their nuclei subsequently pair and 

 fuse. Those without mates, especially those near the periphery, degenerate 

 and disappear. Meanwhile at the surface a wall of several layers has been 

 formed, the suspensors collapse, and the zygospores presently lie free upon 

 their substrate. 



It is characteristic of this process that no dynamic differentiation occurs 

 oetween the + and - energids in spite of their spatial separation. Thus, both 



