CELL. 



[ 146 ] 



CELL. 



parent-cell witliont a cellulose coat, and 

 secrete this afterwards. 



The following modifications occur : — 



a. Division of a nearly solid protoplasm 

 into four, either directly or by two 

 halvings. This occurs in the development 

 of pollen and of the spores of Mosses, Ferns, 

 &c. The parent-cells of the pollen or spores 

 become free in the interior of the anther or 

 sporange, by the solution of the walls and 

 septa of their parent-cells. The protoplasm 

 of the free cells divides into four segments, 

 entirely tilling the ceU. After this, either 

 partitions are formed between these (pollen- 

 cells), to be subsequently dissolved, or they 

 at once clothe themselves with a cellular 

 coat {Marchmitia). In either case, they 

 ultimately lie free in the parent-cell, which 

 is itself finally dissolved (PI. 47. figs. 10-13). 



b. Division of a homogeneous protoplasm 

 into a large number of segments, each of 

 which acquires a cellulose coat, the whole 

 of the new cells lying closely packed but 

 free in the parent-cell. This occurs in the 

 antheridia of the higher Cryptogamous 

 plants, in the formation of the parent-cells 

 of the spermatozoids, also in the formation 

 of the parent-cells of the spores and the 

 elater-ceUs of the Ilepaticse. The formation 

 of the spores in the asci or thecae of the 

 Ascomycetous Fungi and the Lichens be- 

 longs either to this or the preceding case 

 (PI. 37. fig. 12). 



c. Division of the homogeneous proto- 

 plasm into segments which do not acquire 

 a cellulose coat until after they are dis- 

 charged from the parent-cell. Tliis occurs 

 in the development of the zoospores of most 

 of the Coniervoidete {C/adop/ioj-a, Biyopsis, 

 AcJili/a, Ulothrix, &c.), where the primor- 

 dial utricles become free in the cavity of 

 the parent-cell when they divide, and break 

 their way out into the water, where they 

 form a cellulose coat after they have swum 

 about freely for some time by means of 

 their cilia. 



d. Division of a sac-like protoplasm into 

 a number of portions, which appear at first 

 as papillffi on the walls of the cell, and 

 filially become isolated in the cavity. This 

 occurs in the development of the gonidia 

 of Hydrodictyon, Botrydium, &c. These 

 last two cases are connected with a and h, 

 by the circumstance that the zoospores or 

 active gonidia are replaced, under certain 

 circumstances, by cells ; that is, the bodies 

 produced in this way acquire a cellulose 

 coat before they leave the parent-cell. 



Numerous intermediate conditions occur 

 which connect all these together ; and the 

 last case, d, does not difier essentially from 

 what takes place in the formation of the 

 endosperm-cells, placed under 3. 



3. Free Cell- for mat iun. — Here the new 

 cell is formed by a portion of the parent 

 protoplasm separating itself from the rest 

 of the protoplasm, assuming a globular or 

 oval form, and secreting a cellulose mem- 

 brane upon its surface, so as to form a new 

 cell lying free in the cavity of the parent 

 primordial utricle. The most remarkable 

 instance of this case is the formation of the 

 germinal vesicles in the embryo-sac of the 

 Flowering Plants (PI. 47. figs. 1-4) ; but 

 this is denied by Strassburger. Other cells 

 sometimes occur, formed in the same way, 

 at the opposite end of the embryo-sac. 

 The embryo-sac also frequently becomes 

 filled, after fertilization, by a large increase 

 of free cells developed out of the layer of 

 protoplasm or primordial utricle lining the 

 walls; these {endosperm-cells) accumulate in 

 the sac, and sometimes become consolidated 

 into a tissue (albumen) in which the embryo 

 lies imbedded ; in exalbuminous seeds they 

 are re-absorbed during the growth of the 

 embryo. The embryo itself is developed from 

 the germinal vesicle by cell-division such as 

 is described under § 1 (PI. 47. figs. 5, 0). 



Cell-formation also occurs, without divi- 

 sion, in cases where the entire contents of 

 a cell separate fi-om the parent, and form a 

 new organism (rejuvenescence) — as in Sti- 

 yeocloiniim., CEdvyoniiim, and other Con- 

 fervoid Algfe. 



Here also must be referred the new cell- 

 formation resulting from Conjugation. 



Karsten considers that the formation of 

 every cell within a living organ is original ; 

 and that the cell is not divided into two 

 new individuals by transverse septa or pro- 

 liferation. 



The hypotheses of the independent origin 

 of cells from organic substances by generatio 

 a-quivoca seem to require no notice ; but 

 allusion may be made to certain curious 

 phenomena which have been called 'abnor- 

 mal cell-formations,' occurring in some of 

 the Confervoids. The protoplasm of the 

 Siphoneaj is very apt to collect into globu- 

 lar masses in injured filaments; and tluso 

 globular masses apparently acquire a cellu- 

 lose coat in some cases: they have been 

 observed in Vaucheria and Bryopsis ; a some- 

 what similar phenomenon often occurs in 

 the contents of the cells of iSpirogyra. It 



