THE REDUCTION OF THE CHROMOSOMES 



223 



In the RED ALG^E reduction occurs in the two divisions differentiating 

 the nuclei of the tetraspores when the latter are present in the life history. 

 Such is the case in Polysiphonia (Yamanouchi 1906) (Fig. 78, A), Gri- 

 ffithsia (Lewis 1909), and Corallina (Yamanouchi). The brown algae 

 Dictyota (Williams 1904) and Padina (Wolfe 1918) also conform to this 

 scheme. In Nemalion, which has no tetraspores, it was long supposed 

 (Wolfe 1904) that reduction occurs in connection with carpospore forma- 

 tion, .but Cleland (1919) has recently shown that 

 it takes place at the time the zygote germinates, as 

 in so many green algse. 1 



In the ASCOMYCETES reduction occurs in the 

 course of the first two of the three mitoses initiated 

 by the primary ascus nucleus (Figs. 22, 61) and re- 

 sulting in the eight ascospore nuclei. It was for a 

 long time generally thought that there were two 

 nuclear fusions in the life history one in the archicarp 

 and one in the ascus (see p. 290), and the three di- 

 visions in the ascus were accordingly regarded as a 

 process whose function was to reduce the "quadri- 

 valent" chromosomes to the univalent condition 

 (Harper 1905; Overton 1906). Such a double reduc- 

 tion was described by Miss Fraser (1907, 1908) for 

 Humaria rutilans: the first mitosis she found to be 

 heterotypic, the second homceotypic, and the third 

 "brachymeiotic," the last bringing about a further 

 reduction by the separation of the chromosomes 

 into two smaller groups. This was also reported 

 for Otidea aurantia and Peziza vesiculosa (Fraser and 

 Welsford 1908), Lachnea stercorea, Ascobolus furfura- 

 ceus, and Humaria granulata (Fraser and Brooks 1909), 

 and Helvella crispa (Carruthers 1911). Harper (1900, 

 1905), although he thought two fusions occurred, found 

 no double reduction, holding rather that the fusion of the two ascus 

 nuclei and their chromsomes is so complete as to render the quadrivalent 

 character of the latter entirely invisible. Other investigators also find 

 no double reduction in the ascus. They show rather that the first two 

 mitoses correspond to the heterotypic and homosotypic mitoses of other 

 organisms, and that the third division is purely vegetative or equational 

 in character. As instances may be cited the work of Faull (1905, 1912) 



1 For a review of sexual reproduction and alternation of generations in the algae 

 see Bonnet (1914). Davis (1916) gives a convenient summary of the life histories 

 of the red algse. Dodge (1914) summarizes and compares the life histories of red 

 algae and ascomycetes. See Atkinson (1915) for a complete review of researches on 

 ascomycetes. For the cytology of the yeasts see Guilliermond 1920. 



B 



FIG. 78. 



A, prophase of 

 heterotypic division 

 in the tetrasporocyte 

 of Polysiphonia. 

 (After Yamanouchi, 

 1906.) B 



in oogonium of Fucus. 

 (After Yamanouchi, 

 1909.) 



