Myxophycex 



Apart from their interest as very primitive organisms, with a world-wide 

 distribution and a capacity for existence under the most varied conditions of 

 environment, the Myxophycese present a cytological problem which as yet 

 cannot be regarded as entirely solved. Much labour has been expended 

 on the cytology of this group by many skilled investigators, but the conflicting 

 opinions which have been expressed render it a matter of the greatest 

 difficulty to give in a brief space a reasoned account of what might be 

 regarded as our present knowledge of the minute structure of the Myxo- 

 phycean cell. 



Several summaries of this work were given during the years 1903 G, 

 notably those of Kohl ('03), Phillips ('04), Olive ('05), and Guillermond 

 ('06). Subsequently these four papers formed the subject of a critical 

 article by Zacharias ('07). Since then many more investigations have been 

 made with a view to elucidate the rather obscure structural details of the 

 Blue-green Algae, and some of these have been summarised by Pavillard ('10). 



THE CELL-WALL. The cell-wall may be regarded as the definite layer 

 immediately surrounding the protoplast. It is composite in character, 

 and consists in its earlier stages largely of cellulose. Later, it ceases 

 to give any cellulose reactions, offers much resistance to the penetration 

 of reagents, and according to some authors has much in common with 

 fungus-cellulose. Its resistance to reagents caused both Borzi and Hegler 

 to state that there is much resemblance between it and the cuticle of 

 higher plants. Hyams and Richards ('02) have shown that it sometimes 

 contains silica. 



In the adult cell of Oscillatoria and Lyngbya, and in the cells and spores 

 of Anabwna, the cell-wall consists, according to Fritsch 

 ('05), of an ' inner investment,' which is a modified 

 plasmic membi'ane of a viscous gelatinous nature, and 

 a 'cell-sheath/ which is probably the modified inner- 

 most layer of the external sheath of mucilage (fig. 1). 

 Unlike the inner investment, the cell-sheath is soluble 

 in chromic acid except in the mature spore. The cell- 

 sheath would appear to form a coherent whole around 

 the filaments of the Oscillatoriacese, but in the hetero- 

 cystous forms it is split at each cell-division. 



All Blue-green Algse secrete mucilage to a greater 

 or less extent. Most of the colonial unicells, and many 

 of the filamentous forms, are embedded in a more or 

 less extensive mass of mucus, the external surface of 

 which may be covered with a thin cuticle. The 



,-u. 

 c.s 



Fig. 1. Small portion 

 of filament of Toly- 

 pothrix sp. after treat- 

 ment with iodine. x 

 about 1600 (slightly 

 modified from Fritsch) . 

 i.i., inner investment; 

 c.s., cell-sheath; e..s., 

 external sheath. 



gelatinous mass is frequently lamellose (fig. 2, B E), and some or all of 



