mCIvS, ox TlUC DIAMOUPlIOSrS OF LVNGBYA, ETC. 163 



Schizogonium. — When young, filaments simple articulated, 

 with a distinct tube. The cells subsequently by collateral 

 subdivision, produce two, four, or eight parallel rows, formed 

 of a single layer of cells, arranged in siiii|)le or compound 

 lines. 



Prasiola. — Fronds either filiform, formed of a single layer 

 of cells arranged in simple lines ; or membranous, formed of 

 cells arranged in compound lines, or in groups multiple of 

 four. Spores (gonidia) formed of the whole contents of the 

 cells, motionless. 



The only real difference between the first two is, that 

 whereas Lyngbya is a tube containing distinct cells within, 

 which, when old, undergo collateral subdivision, to form a 

 band of two, four, or eight rows of cells ; Schizogonium is a 

 band of two or eight rows of cells, which when young was but 

 a single row, contained in a tube ; which is only two diflFerent 

 ways of stating the same facts. The comparison of the last 

 two is of the same kind. For as Prasiola, when old, is com- 

 posed of many rows of cells, but which arose from a single 

 row, there must have been a time in its life when it had two, 

 four, or eight rows, and thus have been a Schizogonium, for 

 there is no other structural difference between the two. 



Besides, if it be granted that collateral subdivision could 

 extend to two, four, or eight rows, why should it not be 

 admitted, that it could by the same power, go on beyond that 

 limit ? what is there in our knowledge of cell-multiplication 

 to form any obstacle to the supervention of quaternary sub- 

 division, or any multiple of it, upon a binary form. 



The distinctions between Prasiola and Ulva, as insisted 

 upon by Jessen, are equally unstable, both in regard to the 

 regularity of the areas and lines of cells ; as well as to the 

 homogeneousness of their contents. Not only may be seen 

 in Prasiola every variety of form and arrangement of the 

 cells, but sometimes the cells are so close to each other as to 

 obliterate both intercellular lines and the distinction of areas 

 produced by the kind of segmentation. I have seen the cells 

 of Prasiola "granulated''^ throughout the whole frond, as 

 before mentioned (fig. 15), and in the cells Avhich are about 

 to be set free (gonidial spores). I have, at fig. 12 c, shown 

 that they possess a central nucleus. The only difference 

 that seems for the present established between Prasiola and 

 Ulva is, the formation of zoospores in Ulva, and their mode 

 of escape by means of openings in, and not by solution of, 

 the intercellular substance. 



* Ulvarum Q,&\\vi\^ granulatce ; Prtfs/o&n/»z blastemate ^o»?o^^«(?« repletae. 

 Jessen, op. cit , p. 10. 



