202 Mr J. M. Macfarlane 07i the 



agents or forces acting in different directions in the interior 

 of the nucleus ; and what can more naturally supply those 

 than the nucleolus after division has taken place, since 

 each daughter nucleolus now acts as a new centre of in- 

 fluence. It may be asked, have I not been able to observe 

 this most characteristic series of movements in the nuclear 

 fibres of Ornitliogalum during division ? I have not ; but 

 this can be easily explained when we remember that these 

 fibres in young cells are exceedingly delicate ; and, besides, 

 it is not by observing these movements only that we can 

 arrive at a right knowledge of the whole process — a fact this 

 acknowledged by Flemming himself, who confesses that the 

 most obvious phenomena of division which he so carefully 

 discriminates have in all probability the least significance. 

 We will see how well all these propositions can be 

 demonstrated in dealing with Spirogyra. 



Structure and Division of the Cells of Spirogyra nitida, Kg* 



The fresh-water alga Spirogyra is very convenient for 

 studying the structure and division of the cell, since in its 

 cells the nucleus is isolated from the protoplasm, and is 

 connected with the latter only by delicate radiating strands. 

 The particular species which has been chosen for observa- 

 tion is S. nitida, the commonest form in the neighbourhood 

 of our city, and possessing, moreover, a large nucleus, 

 nucleolus, and nucleolo-nucleus. 



1. Structure of the Cells. — Like all the species of the 

 genus it is a filamentous alga, made up of a large number 

 of simple cylindrical cells, joined end to end. Each cell- 

 wall has the usual homogeneous appearance. Immediately 

 inside it, and forming a complete interior lining, is a thin 

 layer of pale, homogeneous, or but slightly granular proto- 

 plasm. Embedded in this are four spirally-arranged bands 

 of chlorophyll. Thanks to the researches of Pringsheim f 

 we are now well acquainted with the structure of these. 

 Each band is studded at intervals with rounded, hollow, or 

 cup-shaped structures, which hold in the cavity a smaller 



* Kiitzing, " Species Algarum," 1849. 



+ Annals and Mag. Nat. Hist., v. 1880. Pringsheim 's " Jahrbiicher," xii. 

 heft 3. 



