BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 221 



ored to accumulate in the following pages all citations which include ref- 

 erences to the occurrence of zygospores among the ]Mucorineae, the litera- 

 ture cited is, no doubt, not wh^^lly complete. It may, however, be 

 assumed to comprise the great majority of citations of this nature. 



An attempt to determine in this literature the thallic conditions of the 

 various species enumerated would be manifestly impossible except in 

 connection with those species in which a homothallic condition was made 

 manifest from the figures or descjiptions. A figure showing the pro- 

 gametes arising ultimately from the same branch would, if correct, be 

 decisive. Unfortunately those who have done most work on this group of 

 fungi and from whom we should expect the most accuracy have erred in 

 describing and figuring a detail which at the time was not considered of 

 importance. Thus, of known heterothallic forms, de Bary ('66, Figure 2, 

 Plate VII) figures Rhizopus, and van Tieghem ('73, Figure 4, Plate XX) 

 and Baiuier ('82, Figure 15) both figure Phycomyces as homothallic; and 

 if Piptocephalis proves to be heterothallic, as is extremely probable from 

 Brefeld's ("72) account of his cultures, this author has figured a similar 

 inaccuracy. That the zygophores can be traced to more or less distant 

 parts of the mycelial growth, though to a certain extent suggestive, is by 

 no means conclusive evidence, since we know that while in homothallic 

 forms the zygophores generally arise comparatively close together, yet in 

 some forms, like Mucors i and ii, the zygophores may arise some little 

 distance apart. In the literature appended, an examination of those spe- 

 cies for which a heterothallic condition has now been made out will show 

 that it is unsafe to place too great dependence on the accuracy of the 

 figures and descriptions of morphological characters as published. 



But aside from the morphological data, the action of the fungus in 

 cultures may become circumstantial evidence of considerable value. If 

 cultures from a single spore or from a single sporangium always fail to 

 produce zygospores, while in cultures from a mass of sporangia zygo- 

 spores appear, the evidence even without morphological facts points al- 

 most certainly to a segregation of sexual characters on separate mycelia. 

 In most of the cases cited, zygospores were found by chance, and no fur- 

 ther cultures were made, so that the evidence from the action in cultures 

 is lacking. It has not infrequently happened, however, that zvgospores 

 have been found after a sowing on some unsterilized medium which is a 

 common source of the species investigated, and subsequent attempts to 

 continue the growth of zygospores by transfers of spores has resulted in 

 failure. An accidental mixture of sexual strains is the only available ex- 

 planation of the failure to obtain zygospores in one culture of a given 



