CELL. 



[ 121 ] 



CELL. 



tain size, its primordial utricle detaches itself 

 from that of the parent, and a partition is 

 formed at the point whence the second cell 

 emerged(Pl. 20. fig.23). 



Another point which must be noticed here, 

 is the question whether the parent primor- 

 dial utricle divides instantaneously , at a given 

 epoch, into the new utricle, or whether it 

 parts gradually, by a sort of constriction ad- 

 vancing from the surface towards the centre, 

 roughly comparable to what occurs when a 

 ligatm-e is slowlv drawn tight round an elastic 

 tube, or when a bar of soap is cut in two by 

 passing a string round it and gradually draw- 

 ing the loop tight. It seems probable that 

 the segmentation of the primordial utricle is 

 always gradual, and it is certain that it is so 

 in many cases. Its gradual constriction has 

 been oljserved in those Confervse where the 

 primordial utricle is a hollow sac, forming a 

 lining over the whole internal surface of the 

 parent-cell ; it may be traced in the larger 

 ConfervcE, in Spirogyra, &c., by keeping the 

 plants growing in water under the micro- 

 scope. It appears that the division is gene- 

 rally completed during the earlier hours of 

 the morning. 



2. Cell-division with liberation of the new 

 cells. — The first step in this process is ana- 

 logous to what takes place at the outset in 

 the preceding set of cases; but we find much 

 more important modifications here. This is 

 the mode of development of spores of the 

 Ascomycetous Fungi, of the spores andtetra- 

 spores of the Algae, the spores of Lichens, 

 the spores of all the higher Cn'ptogamia, 

 the active gonidia or zoospores of the Algse, 

 the parent-cells of the spermatozoids or active 

 spiral filaments of the higher Cryptogamia, 

 and of the pollen grains of the Flowering 

 plants. 



The general character is : Division of the 

 whole primordial utricle into segments, 

 which either acquire a cellulose coat within the 

 parent-cell before they are set free by its so- 

 lution or bursting, or escape from the parent- 

 cell without a cellulose coat, and secrete this 

 afterwards. 



The following modifications occur : — 



a. Division of a nearly solid primordial 

 utricle 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 primordial 

 utricles of the free cells divide into four seg- 



ments, entu'ely filling the cell. After this, 

 either partitions are formed between these 

 (pollen-cells), to be subsequently dissolved, 

 or they at once clothe themselves with a 

 cellular coat {Marchantia). In either case, they 

 ultimatelv lie free in the parent-cell, which 

 is itself finally absorbed (PI. 38. figs. 10-13). 



b. Division of a homogeneous primordial 

 utricle 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 

 antheridiaof 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-cells 

 of the Hepaticse. The formation of the 

 spores in the asci or thecse of the Ascomyce- 

 tous Fungi and the Lichens, belongs either 

 to this or the preceding case (PI. 29. fig. 12). 



c. Division of the homogeneous primordial 

 utricle into segments which do not acquire a 

 cellulose coat until after they are discharged 

 from the parent-cell. This occurs in the 

 development of the zoospores of many of the 

 Confervoidese ( Cladophora, Bryopsis,Achlya, 

 Ulothrix, &c.), where the primordial utricles 

 become free in the cavity of the })arent-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 primordial utricle 

 into a number of portions, which appear at 

 first as papillse on the walls of the cell, and 

 finally become isolated in the cavity. This 

 occurs in the development of the gonidia 

 of Hydrodictyon, Botrydium, &c. These 

 last tw o cases are connected with a and b by 

 the circumstance that the zoospores or active 

 gonidia are replaced, under certain circum- 

 stances, 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, is closely related to w hat takes 

 place in the formation of the endosperm-cells, 

 placed under 3. 



3. Free Cell-formation. — Here the new cell 

 is formed by a portion of the parent primor- 

 dial utricle separating itself from the rest of 

 the protoplasm, assuming a globular or oval 

 form, and secreting a cellulose membrane 

 upon its sm'face, so as to form a new cell 

 lying free in the cavity of the parent primor- 

 dial utricle. The most remarkable instance 

 of this case is the formation of the germinal 



