590 PHYSIOLOGY. 



they become so many free cells, which escape by the bursting or solution 

 of the parent cell. 



This case occurs frequently, with the production of four cells, in the 

 development of pollen-grains and the spores of the higher Cryptogamia. 

 Sometimes there is a certain irregularity in such cases, the parent cell 

 either becoming really chambered by cell-division, and forming one cell 

 in each of the four chambers, or at once giving birth to the four free 

 daughter cells. 



Other instances of this modification are found in the development of the 

 spores of the asci of Lichens and Mosses, apparently also in the tetra- 

 spores of Florideae. The development in this way of new cells which 

 escape from the parent cell as naked utricles is observed in the spores of 

 Fucus (fig. 595), and in the zoospores of many Confervoids, which are 

 formed in fours in each cell (Ulva, Coleochtete, &c.). 



The resolution of the whole contents of a cell into a great number of 

 free cells occurs in the formation of the very numerous zoospores of Cla- 

 dopliora (fig. 512, 0, d) and Achlya, with the formation of the new cell- 

 membrane after their escape from the cavity of the parent ; and what is 

 observed in these cases leads to the conclusion that a similar mode of de- 

 velopment, going on to the completion of the cells within the parent, 

 occurs in the formation of the parent cells of the spermatozoids or anther- 

 idia of the higher Cryptogamia, where a great number of minute free 

 cells are developed, and are found free in the cavity of a large parent cell. 

 The formation of the new fronds of Ilydrodictyon is a remarkable case of 

 the resolution of the whole contents of a cell into a vast number of free 

 cells, which acquire their cellulose coats and cohere into a new network 

 within the parent cell. 



In the formation of the germinal vesicles in the embryo-sac of the 

 Phanerogamia, and probably in the cell corresponding to the embryo-sac 

 in the archegonia of the higher Cryptogamia, a portion only of the proto- 

 plasmic substance of the parent cell takes part, becoming isolated in the 

 form of one or more (usually three in Phanerogamia) globules (fig. 594, A), 

 one (or sometimes two) of which acquires a cellulose membrane and 

 forms the first cell of the embryo, or its suspensor (fig. 594, B). The new 

 cell is here often very much smaller than the parent cell ; and this case 

 thus offers the clearest and most striking instance of free-cell formation. 



In the embryo-sac of many of the Phanerogamia we observe, subse- 

 quently to impregnation, a process of free-cell formation of a peculiar 

 kind, the protoplasm of the embryo-sac breaking up by degrees into nume- 

 rous corpuscles, which successively form cellulose coats, and apply them- 

 selves to the wall of the embryo-sac, until the layers meet in the centre, 

 and the whole sac is filled up with a parenchymatous tissue, the cells of 

 which (endosperm-cells) are at first very loosely coherent. Perhaps the 

 parent cells of the spermatozoids are formed in this way, in the cells of the 

 antheridia of the higher Cryptogamia. 



The formation of the active zoospore of VaucJieria is really a result of 

 the isolation and individualization of a portion of the contents of the 

 parent cell, since here the whole plant is one gigantic cell ; but this 

 case is quite different from the developments included in the preceding 

 paragraph. 



Free-cell formation occurs either with or without the presence of a 



