THE REDUCTION OF THE CHR0M0S0M1 





In the red alg^e reduction occurs in the two divisions differentiating 

 the nuclei of the tetraspores when i he Latter are presenl in t be life history. 

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

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

 Dictyota (Williams 1904) and Padina (Wolfe L918) also conform to this 

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

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

 tion, but Cleland (1919) has recently shown thai 

 it takes place at the time the zygote germinates, as 

 in so many green alga?. 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 arch i cat}) 

 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- 

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

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

 1905), although he thought two fusions occurred, found 

 no double reduction, holding rather that the fusion of tin- two a>ens 

 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 homceotypic mitoses of other 

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

 in character. As instances may be cited the work of l'aull I 1905, 1912 



1 For a review of sexual reproduction and alternation of generations in tin- algffi 

 see Bonnet (1914). Davis (1916) gives a convenienl summary of tin- life histories 

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

 alga? and asconivccU's. See Atkinson I I'M.", for a complete review <>t' researches on 

 ascomycetes. For the cytology of the yeasts ruilliermond 1920. 



Fio. 78. 



.1, prophase <>f 

 heterotypic division 

 in the tetrasporocyts 

 of /' o I ys i }> h i) ri ia . 

 (After Yamanouchi, 

 1906.) Ji, proph 

 of heterotypic mit 

 in < oogonium of I 

 (After Yatnai 

 L909.) 



