FORMATION OF CELLS. 
9 
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 i-egarded 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, «), which continue to grow 
until they meet {h). 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, 0), 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 
tiG. \.~A, B escape of the swarm-cells of an 
(Edogoiiium; C one free in motion; Z> tlie same 
after it has become fixed and has formed the 
attaching- disc ; E escape of the whole protoplasm 
of a young- plant of CEdo£-07iiu7n in the form of a 
swarm-cell (X3S0). (After Pringsheim, Jahrb. für 
wiss. Bot, vol. I. pi. I.) 
^ [Strasburger, lieber Befruchtung und Zelltheilung, 1878, The protoplasm of one cell, the one 
the protoplasm of which passes over into the other, usually contracts first,] 
