12 



genei-ally been neglected. Tlie dimensions of tlie cells and nuclei are 

 so much smaller than those in the larger species of Spirogyra, that 

 one would expect to meet with still greater difficulties in a karyo- 

 kinetic investigation than would arise in the case of Spirogyra. 

 Such is indeed the case and in studying karyokinesis I have not 

 been able to trace the details of the process to the same extent as 

 in different species of the genus Spirogyra. 



Mabei. L. Merkiman M was the first to study karyokinesis in 

 Zygnema. She could not with certainty identify the species studied, 

 because she had no zygospores at her disposal. The chief results of 

 her in({uiry were as follows. 



She found in the nucleus of Zygnema no body that corresponds 

 to the nucleoli of higher plants. There is in the middle of the 

 nucleus a central body that is composed of the greater portion of 

 the cromatin-granules, whilst the rest of the chromatin-granules is 

 situated in the peripheral network between the central body and 

 the nuclear membrane. During karyokinesis the central body splits 

 into many small ones, whilst the granules in the network increase 

 in size. In this way there are formed 20 or more mostly loose 

 chromosomes. A spirein is not formed. The chromosomes come to 

 lie in a ring round the centre. The nuclear membrane dissolves. 

 Then the chromosomes approach one another, and unite into 4 to 6 

 tetrads or groups of four, which become arranged in two parallel 

 planes, lying close together. The chromosomes of these two planes 

 separate. No longitudinal splitting takes place. The groups of four 

 now divide into smaller groups, which form two rings. Thereupon 

 the central body is formed, composed usually of the greater number 

 of the chromosomes. A nuclear membrane also appears again. 

 Daughter nuclei with many tetrahedral granules, with several masses 

 and with a single mass are observable. 



EuD. EscoYEZ ') investigated the nucleus and karyokinesis in a 

 species of Zygnema, which he believes to have been different from 

 that studied by Miss Merkiman, but which he could not identify. 



His results are entirely different from those of Miss Merriman. He 

 states that in the resting nucleus, there can be distinguished a net- 

 work, an ordinary nucleolus and a nuclear membrane. Rarely there 

 are two nucleoli in the nucleus. The nucleolus, according to Escoyez, 

 lies in a cavity (cavité périnucléolaire) which is surrounded by a 



1) Mabel L. Merriman, Nuclear division in Zygnema, Reprinted from The Bo- 

 tanical Gazette, 41, Jan. 1906, p. 43-53. 



2) EuD. Escoyez, Le Noyau et la Garyocinèse chez le Zygnema, Extrait de la 

 Revue «La Cellule", t. XXIV, 2d fasc. 1907, p. 365-367. 



