376 



SCIENCE 



[-N. S. Vol. LI. No. 1320 



observed sexual reproduction of the bread 

 mold and its relatives that I have chosen as 

 my theme. 



It will not be possible in the time avail- 

 able to enter into any detailed discussion of 

 many questions that might suggest them- 

 selves in this connection. I shall instead give 

 an outline merely of some of the investiga- 

 tions of the last fifteen years, both published 

 and unpublished and shall attempt to show 

 that the sexual relations of the mucors may 

 have possible bearings upon our conceptions 

 of sexuality in other diverse groups of the 

 biological world. 



The chart (9, Tafel VI.)^ shows the typical 

 vegetative condition of a mucor. A vegeta- 

 tive spore, usually multinucleate though some- 

 times iminucleate, sends out in germination a 

 branching tube which forms the mycelium 

 and rapidly covers the available substratmn. 

 This mycelium is multinucleate and, in the 

 early stages at least, without cross walls — 

 forming thus an enormous, much-branched, 

 single cell — if a cell is defined in terms of 

 the limiting cell walls. Multiplication is 

 brought about chiefly by various types of non- 

 sexual spores. The commonest are endog- 

 enous spores produced in sporangia, upwards 

 of 70,000 individual spores being formed in a 

 single sporangium. They may be apparently 

 exogenous, formed singly or in chains on 

 terminal swellings of fertile filaments and 

 may be produced as chlamydospores by septa- 

 tion of the vegetative filaments. More than 

 a single type of nonsexual multiplication may 

 occur in a given species. 



As regards their sexual reproduction, there 

 are two groups of species. In the first group, 

 represented by Sporodinia, a form common 

 on fleshy fungi, the sexual spores known as 

 zygospores are common and may be obtained 

 from the sowing of a single vegetative spore. 

 Such forms are therefore hermaphroditic or 

 homothallic since their thalli or mycelia are 

 alike sexually. In the second group, repre- 



2 Citations in parentheses throughout the text 

 refer to the sources for the charts and lantern 

 slides used in the original presentation of this 

 paper. 



sented by Bhizopus, the bread mold, the 

 zygospores are rarely observed and can never 

 be obtained in pure cultures from the sowing 

 of a single spore. In these diecious or hetero- 

 thallic forms there are needed two plants of 

 opposite sex growing in contact in order that 

 sexual reproduction may take place. The two 

 sexual groups mentioned are represented in 

 the adjoining diagram (Fig. 4). Since in 

 the three lower figures the two gametes 

 (which later unite to form the mature zygo- 

 spore) arise from branches of a single fila- 

 ment, these three forms are hermaphi'oditie. 

 In the upper figure the gametes are repre- 

 sented arising from sexually different plants 

 designated by the signs plus and minus 

 which will be explained later. These there- 

 fore belong to the diecious group. The line 

 of zj'gospores, which results when the opposite 

 sexes of the diecious species Mucor Mucedo 

 are grown in contact, is shown in the chart. 

 The swollen heads produced on erect filaments 

 from the plant on the right of the line are 

 sporangia containing numerous nonsexual 

 spores by which the plant may be propagated 

 as distinct sexual races in much the same 

 manner in which races of potatoes may be 

 propagated by non-sexual tubers. The process 

 of conjugation may be followed from the 

 figures in the chart (9, Tafel YII.). Fila- 

 ments of opposite sexual tendencies grow to- 

 gether and by the stimulus of contact produce 

 swellings which push them apart. These 

 swellings develop into the progametes from 

 which by cross walls the sex cells, or gametes, 

 are cut off. The dissolution of the interven- 

 ing cross wall allows a fusion of the gametes 

 and the zygote thus formed increases in size 

 and becomes the mattu-e zygospore. The 

 gametes are typically equal in size in the 

 diecious group and also in the hermaphroditic 

 group except for certain fonns to be dis- 

 cussed later. They are miltinucleate and 

 hence have been called coenogametes. 



In the first lantern slide (1, PI. IV.), we 

 can see photographs of Petri dish cultures of 

 certain of the mucors experimented with. 

 The opposite sexes of the diecious species 

 have been termed plus and minus for reasons 



