68 GENERAL niSTORY OF THE INFUSORIA. 



phenomenon of the formation of a tubular membrane, often impregnated with 

 silex, and elegantly dotted or areolated, connecting the two segments of 

 Isthmia, Melosira, &c., depends upon the same process. 



" An analogous case is met with in the formation of the spores of Pellia 

 epipliylla. The mother- cell here produces six ridges of cellulose projecting 

 inward from the internal wall, intersecting at an angle of 60° ; these ridges 

 grow in toward the middle point of the cell, like the annular ridge of Cla- 

 dojpJiora at the commencement of cell-division. A\Tien these projecting ridges 

 have attained the breadth of a fourth part of the transverse cHameter of the 

 mother- cell, the cell- contents divide into four parts, which, retracting from 

 one another and from those ridges, occupy the four chambers of the cell, each 

 of which is vaulted externally and bounded laterally by three of the ridges, — 

 here becoming coated with a membrane and developed into a spore, while the 

 tetrahedi'al space in the middle of the cell, bounded by the six ridges, remains 

 filled only with watery fluid. The spores become free by the solution of the 

 enveloping part of the membrane of the mother- cell. The resemblance of 

 this process to the vegetative multiplication of Navicula consists in the inter- 

 ruption of the division of the cell by the formation of septa, and the subse- 

 quent completion of the daughter- cells by secretion of membrane on the 

 external surface of contracted portions of the contents of the mother- cell. 

 A deviation occurs in the circumstance that in Pellia the segment of the coat 

 of the mother- cell which is in contact with the external smface of the daughter- 

 cell becomes dissolved, while in Navicula it persists and remains most inti- 

 mately connected with the daughter- cell. 



" The newly-formed parts of the cell-coat facing together in the division 

 are, in the Diatomeae, and still more clearly in the Desmidiea), perfectly smooth 

 and even for some time after theii' production ; it is subsequently that they 

 obtain the often veiy considerable tubercles and spines, consisting principally 

 of cellulose. The same applies to the processes upon the outer integument of the 

 spores of Euastra, Cosmaria and Staurastra produced in the conjugation. These 

 phenomena, as also the autumnal secretion of jelly by many of the Desmidiese, 

 deserve more notice than they have hitherto attracted in connexion with the 

 theory of the life of the vegetable cell. Still more remarkable behaviour is 

 displayed by the cell-coat of an organism which I refer only doubtfully to 

 the DesmidieEe. In many pools about Leipsic, in which Desmidiese abounded, 

 occuiTcd large, accurately si^herical, tliick- walled cells, some as much as -05 

 miUim. in diameter, rich in chlorophyll, which not only lined the internal wall 

 as a connected granular layer, but — as in many Desmidieae — formed groups, 

 distributed, in the interior of the cell, in a system of radially-arranged plates, 

 which presented a stellate appearance when seen from the side. It would 

 be no great stretch of imagination to regard these cells as the conjugation- 

 spores of a large Desmidiean. But these spores are all spiny, with the single 

 exception of those of Xanthidium armatum. This very striking form occurs 

 but rarely with us, having hitherto been found only in a single locahty, while 

 these globules are as common as they are abundant, and are often found in 

 great numbers in forest pools, wliich harbour, in addition to them, only very 

 small Desmidieae. But such a supposition is still more decidedly negatived 

 by the circumstance that the cells in question are sometimes found dividing 

 into two. This renders it in the highest degree probable that they are inde- 

 pendent organisms — Desmidieae without a central constriction, which may 

 form the commencement of a series of forms terminating in Micrasterias. 



" These cells frequently appear surrounded by a wider coat, inside which the 

 cell then floats freely, enclosed by its own closely-investing coat. Several 

 such empty coats are often met 'vvith, even as many as six sticking one inside 



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