618 AQUATIC PHYCOMYCETES 



by the fact that the orientation of the gamete nuclei in the zygote is 

 such that their still intact nuclear caps do not, as is ordinarily the case 

 form a barrier to fusion. In most instances the nuclear caps of the two 

 gametes merge soon after conjugation. The initiation of zygote germi- 

 nation is apparently not dependent upon the fusion of the gamete nuclei, 

 since in about one half of the specimens examined karyogamy had 

 occurred at this time and in the remainder it had not. It is, however, 

 dependent upon dissociation of the fused nuclear cap, for germination 

 is never initiated until this is completed. He asserted, therefore, that 

 the fragmentation of the nuclear cap is the factor causing the zygote 

 to start its development. 



In summarizing the results of a cytological study of gametogenesis 

 in AUomyces arbuscida, Hatch (1935) stated: 



1. In the hyphal tips of A. arbuscula there are numerous nuclei, lipoid 

 granules, and chondriosomes. Nuclei and lipoid granules are distributed at 

 random; the chondriosomes are concentrated in the tip. The nuclei have six 

 chromosomes; the lipoid granules are grey-black in colour, small and spher- 

 ical; the chondriosomes are long and filamentous. 



2. When the male and female gametangia are cut off from a hyphal tip the 

 nuclei and lipoid granules are distributed between the male and female game- 

 tangium in approximately equal numbers. The chondriosomes segregate un- 

 equally, a disproportionately large number going to the terminal, female 

 gametangium. 



3. In early gametogenesis the nuclei divide more often in the male game- 

 tangium, so that it contains roughly twice as many nuclei as the female. 

 These male nuclei are one-half the size of the female nuclei. The chromosome 

 count, however, is six in the nuclei of both gametangia. These nuclei become 

 the "organization centres" in the formation of gametes, and each collects 

 about itself a sheath of lipoid granules and a tangled mass of fine, filamentous 

 chondriosomes. 



4. In late gametogenesis the lipoid granules in the male gametangium 

 change colour, becoming salmon-pink, while those in the female remain grey. 

 The chondriosomes fragment into a cloud of small granules, and these gran- 

 ules subsequently enlarge and fuse to form, first, a reticulate mantle about the 

 nucleus, and finally, a single large mass which becomes appressed to the nu- 

 cleus in the form of a nuclear cap. 



5. The female gamete is two to three times as large as the male; its nucleus 

 is twice, and its nuclear cap three to four times, as large as that of the male. 

 Its lipoid granules are grey, while those of the male are of a brassy colour. 



6. Since the nuclei in the hyphae are genetically the same and are segregated 



