FORMATION OF CELLS. 



formed on the side (Fig. 4, A, E), not at one end of the protoplasm -mass. The arrange- 

 ment of the entire protoplasm of the cell is therefore entirely changed ; the trans- 

 verse becomes the longer diameter of the cell and of the plant arising from it. The 

 material remains, as far as can be seen, the same, but its arrangement is different. 

 This is the point of morphological importance, that every formation of a new cell 

 depends essentially on a fresh arrangement of protoplasm already in existence. Hence 

 the rejuvenescence of a cell not only may but must be regarded morphologically as the 

 formation of a new one. 



2. Cell-formation by Conjugation. — The protoplasm of two or more cells 

 coalesces to form one common protoplasm-mass, which surrounds itself with a cell-wall 

 and becomes endowed with the other properties of a 

 cell. To study this process, which presents many 

 variations, we may take one of our commonest fila- 

 mentous Algae, Spirogyra longata (Figs. 5, 6). Each 

 filament (Fig. 5) consists of a row of similar cylin- 

 drical cells, each of which contains a protoplasm-sac ; 

 this encloses a relatively large quantity of cell-sap, in 

 the midst of which hangs a nucleus, enveloped in a 

 small mass of protoplasm, and attached to the sac 

 by threads of the same substance; in the sac lies a 

 spirally coiled chlorophyll-band, with thickenings 

 (chlorophyll -granules) at intervals which contain 

 starch-grains. The conjugation always takes place 

 between opposite cells of two more or less parallel 

 filaments. The first stage is the formation of lateral 

 protuberances (Fig. 5, a), which continue to grow 

 until they meet [b). The protoplasm of each of 

 the two cells concerned then contracts^, detaches 

 itself from the surrounding cell-wall, rounds itself 

 into an ellipsoidal form, and contracts still more by 

 expulsion of the water of the cell-sap. This may 

 occur simultaneously in the two conjugating cells. 

 Next, the cell-wall opens between the two protuber- 

 ances (Fig. 6, a), and one of the two ellipsoidal 

 protoplasm-masses forces itself into the connecting 

 channel thus formed, gliding slowly through it into 

 the other cell-cavity ; and as soon as it touches the 

 protoplasm-mass contained in it, they coalesce (Fig. 

 6, a). After complete union (Fig. 6, b) the united 

 body is again ellipsoidal, and scarcely larger than 

 one of the two from which it was formed ; during 

 the union a contraction has evidently taken place, 

 with expulsion of water. The coalescence gives the 

 impression of a union of two drops of fluid; but 

 the protoplasm is never fluid in the physical sense of 



the word. The conjugated protoplasm-mass clothes itself with a cell-wall, and forms a 

 Zygospore, which germinates after a period of repose of some months, and then developes 

 a new filament of cells. With greater or smaller deviations from this plan, conjugation 

 takes place in a large group of Algae, the Conjugatae, among which the Diatoms must be 

 included, and in some Fungi, the Zygomycetes. In the latter more considerable de- 

 viations occur. In Spirogyra nitida it also happens (De Bary, Conjugaten, p. 6) that one 



Fig. 4-—^, B escape of the swarm-cells of an 

 (Edo^otiium; C one free in motion; /) the same 

 after it has become fixed and has formed the 

 attaching disc ; E escape of the whole protoplasm 

 of a young plant of (Edogonium in the form of a 

 swarm-cell (X350). (After Pringsheim, Jahrb. fur 

 wiss. Bot. vol. I. pi. r.) 



^ [Strasburger, Ueber Befruchtung und Zelltheilung, 1878. The protoplasm of one cell, the one 

 the protoplasm of which passes over into the other, usually contracts first.] 



