210 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Progametes may either arise from uudifFereutiated aerial liyphae, as 

 in Rhizopus, or may originate as in M. Mucedo, from special branches, 

 which, rising from the mycelium, are mutually attracted and develop their 

 progametes at the point of contact. It will thus be convenient to distin- 

 guish two ty])es of more or less distinctly differentiated fertile hyphae ; 

 namely, sporangiophores bearing nonsexual spores in sporangia, and 

 zygophores, which give rise to progametes and zygospores, although this 

 distinction is not always well marked and both types of spores may 

 originate from the same hypha. The successive steps in the i>rocess of 

 conjugation as it occurs in Mucor Mucedo is represented in Plate II, 

 Figures 25-35. 



With the exception of de Bary's ('66) early description of Rhizopus, 

 which is not in accordance with his later statements ('84, p. 159), and 

 Falck's ('01, p. 241) description of Sporodinia, which, however, he was 

 unable to confirm by observation, writers on the Mucorineae, in describ- 

 ing the process of conjugation, have stated that club-shaped progametes 

 develop from opposite sides of hyphae not already in contact, and, 

 increasing in size, grow through mutual attraction to meet each other at 

 their swollen extremities. This account, illustrated with figures, has been 

 generally current in text-books which mention the subject. However, in 

 the species in which the process has been observed by the writer, includ- 

 ing both homo- and hetero-thallic forms, the progametes are from the 

 very first always normally adherent and arise as the result of the stimu- 

 lus of contact between the (-1-) and (— ) zygophoric hyphae which in 

 some cases at least have been shown to be mutually attractive, and may 

 therefore be termed zygotactic. 



Although there is in general little if any diflferentiation in the conju- 

 gating apparatus which might possibly indicate a sexual diflferentiation, 

 yet two genera of the homothallic group — Zygorhynchus and Dicrano- 

 phora — are heterogamic in that their gametes show a certain constant 

 inequality in size which corresponds to even a greater difference in size 

 and form of their suspensors and zygophores. All other species of this 

 group known are apparently isogamous. 



In the heterothallic forms, where we should perhaps most expect to 

 find a structural difference in the conjugating cells, the gametes are 

 morphologically equivalent, so far as has been determined. DiflTerences 

 in the size of the gametes or of the suspensors, as, for example, in Rhizo- 

 pus, and priority of one suspensor over the other in developing out- 

 growths, as in Phycomyces, although considered by some authors as 

 indicative of a sexual diflPerentiation, are inconstant, and will be subse- 



