PHYSIOLOGY. 



perceptible ; but it appears as though in some instances they became 

 transformed into a kind of cement, gluing the cells together, but capable 

 of being dissolved by nitric acid so as to set the wood-cells free. 



Cell-division occurs as a forerunner of free-cell formation in many 

 cases, when a tissue is about to give birth to a great number of free cells ; 

 as in the formation of the pollen-grains in anthers, and the spores in the 

 sporanges of the higher Cryptogamia, where the structure is in the first 

 instance developed into a quantity of chambers by cell-division, each of 

 the compartments then producing- a free cell. 



This must be borne in mind presently, when we come to speak of the 

 modifications of free-cell formation. 



Changes in the Nucleus. During the division of the protoplasm 

 as above described, and in intimate connexion with it, complex 

 changes occur in the nucleus, which have been specially studied by 

 Strasburger in Spiroc/yra. At the commencement of cell-division 

 (which occurs only at night) the nucleus increases in size, loses its 

 nucleolus, alters in form, and shows in the centre a transverse 

 constriction in the form oE a band, the nucleus-band, which is 

 marked by rod-like condensations of protoplasm. Subsequently 

 the nucleus, still increasing in size, again alters its form, becoming 

 barrel-shaped, with an aggregation of granular protoplasm at each 

 end. Division takes place across the nucleus-band in about 15 

 minutes after its first appearance. Two new nuclei are formed, 

 which separate partially one from the other, but are connected by 

 means of protoplasmic threads with the aggregations of protoplasm 

 at the two ends of the parent nucleus. Other threads gradually 

 connect the new nuclei with the protoplasm lining the walls of the 

 cells, and from which the septum is formed. This septum passes 

 between the two nuclei, and divides the original cell into two, each 

 with its nucleus. During the subdivision of the nuclei, nucleoli 

 are formed in them, but these disappear with the exception of one. 

 In Spiroyyra the chlorophyll is arranged in ribbon-like coils (see 

 fig. 549), and along these coils, when the cell and nucleus subdivi- 

 sion are taking place as just described, currents of protoplasm, 

 carrying starch-grains along with them, may be seen ; so that about 

 three quarters of an hour after the first change in the nucleus, a 

 ring of protoplasm is found to extend round the cell, towards which 

 the starch-grains proceed in great numbers. On this ring the first 

 layer of cell-membrane is formed at the expense of the starch- 

 grains. The nucleus which, just prior to division of the cell, is 

 placed near its centre, is placed, after that process is completed, 

 near the side. 



Free-cell formation. The essential character of free-cell for- 

 mation lies in the circumstance that the protoplasm which produces 

 the primary cellulose wall of the new cell previously becomes sepa- 



