192 INTRODUCTION TO CYTOLOGY 



not to have been reached. To what extent the apparently simpler types 

 of nuclear organization and of division ("promitosis") represent stages 

 in the evolution of more complex types must remain for the future to 

 determine. 



Blue-green Algae. — The structure of the cell in the Cyanophycese 

 has long been a subject of controversy. ^^ The chief question at issue has 

 been the nature of the central region, which appears relatively colorless 

 and ill defined in living material. By some observers it has been denied 

 that this in any way corresponds to a nucleus, the contention being that 

 blue-green algse, like bacteria, have a cell organization fundamentally 

 different from that of other organisms. Others have maintained that 

 the central region, although it is not sharply delimited, may be viewed 

 as a "primitive nucleus," since it is occupied by a material which often 

 stains like "chromatin" and tends to be distributed rather equally to 

 the daughter cells during cell-division'^" (Fig. 113). It is reported 

 that in some genera it contains a material reacting positively to the Feul- 

 gen test, also that in Nostoc no nucleic acid can be demonstrated 

 although its radicles are present. ^^ The degree of distinctness with 

 which such "nuclear" materials are aggregated in the central region 

 varies widely. 



At present it is difficult to place a proper evaluation on the conflicting 

 claims made in this field. Many have provisionally concluded with 

 Guilliermond that in at least some blue-green algae there is a primitive 

 nucleus consisting mainly of chromatic matter often arranged in strands, 

 that such strands are broken as the whole mass divides by a process some- 

 what intermediate between amitosis and mitosis, and that no distinct 

 membrane appears about the mass because the latter is almost constantly 

 in process of division. Definite chromosomes and longitudinal division 

 of chromatic strands have not been demonstrated. 



The peripheral cytoplasm in such cells with a central body contains 

 chlorophyll, together with phycocyanin or phycoerythrin. The exact 

 manner in which these pigments are borne is uncertain,^- but it is thought 

 probable that they occur in numerous minute droplets which may some- 



23 Among cytological works on the Cyanophyceae are those of Biitschh (1896), 

 Fischer (1897, 1905), Hegler (1901), Kohl (1903), Olive (1904), Phillips (1904), Gard- 

 ner (1906), Guilliermond (1906, 19256, 1926), W. H. Brown (1911a), Acton (1914), 

 Baumgartel (1920), Haupt (1923), S. Lee (1927), Pascher (1929), and Prat (1925). 

 Convenient resumes are given by Olive and Haupt. Lloyd (1924) has given special 

 attention to the pigments. 



™Kohl (1903) on Tohjpothrix; Olive (1904) on Oscillatoria; W. H. Brown (1911) 

 on Lyngbya; Guilliermond (1906) on Phormidium, Nostoc, and Rivularia; Haupt 

 (1923) on Anabcena and Gloeocapsa. 



^' Poljansky and Petruschewsky (1929) on Oscillatoria, Tolypothrix, Spirulina, and 

 Gloeotrichia ; Mockeridge (1927) on Nostoc. 



^^ See the discussion of fluorescent colors by Lloyd (1924). 



