162 PHYCOMYCETEAE 



occasionally, a few spores that are not yet differentiated sexually and 

 which produce homothallic plants. Even in these plants the spores in the 

 sporangia that they bear become more and more differentiated into the 

 two sexes. Evidently this is connected with the nuclear condition found 

 by Cutter to exist in the zygospores of this species. Sjowall (1945) claims 

 that in M. mucedo and M. hiemalis the gamete nuclei unite and undergo 

 meiosis early in the zygospores so that for most of the resting period the 

 nuclei are haploid. These nuclei undergo degeneration until finally only 

 a few, apparently all of the same sexual phase, enter the sporangium. In 

 Rhizopus the union of nuclei occurs as a zygospore is germinating and 

 the reduction division as the sporangium is being formed, evidently only 

 one diploid nucleus being concerned. According to him in the process 

 of meiosis three of the four nuclei formed degenerate so that only one 

 haploid nucleus is available for further functioning. In the sporangium 

 the spores start out with one nucleus each, usually, but in many cases 

 this divides so that the mature spores have several nuclei. 



The sexual process of the Mucor type is as follows : At a point where 

 the hyphae come in contact (if of a heterothalhc species these two hyphae 

 must originate from plants of opposite sex) a swelhng occurs in each 

 hypha, pushing them apart. These two swellings flatten against one 

 another and become much enlarged, tapering down to the hyphae from 

 which they arose. Soon a cross wall appears in each of the two processes, 

 parallel to and a little distance from the flattened surface of contact, 

 forming the two gametangia. No significant difference in the size of the 

 two gametangia can be noticed except in a few species. Each contains a 

 rather dense mass of cytoplasm with many nuclei. Beginning at the center 

 the double wall separating the two gametangia dissolves away. In some 

 cases there is a bulging of the central part of this double wall into one 

 of the gametangia and when the wall is dissolved the cytoplasm and 

 nuclei pass into this receptive gametangium. Soon, however, the whole 

 intervening wall is dissolved away and the mingled cytoplasm fills out 

 the cavities of the two gametangia. In the very young zygospore the thin 

 wall represents that of the original gametangia. Within this is laid down 

 a layer of more or less distinct dark plates or pyramids below which a 

 two- or three-layered cell wall is secreted. As the zygospore enlarges the 

 original gametangial wall is broken into small pieces that flake off so 

 that finally the exterior is covered by the dark plates which may re- 

 semble short broad spines. Ling- Young showed that in many cases of 

 heterothalhc species the more vigorous strain (female) usually produces 

 a larger gametangium and a larger suspensor than does the male my- 

 celium. In Mucor hiemalis Wehm. not only is this the case but according 

 to this author the contents of the male gametangium pass into the female 

 gametangium through a small opening which then becomes closed so 



