CELL DEVELOPMENT. 351 



when this occurs, the cell may be simply solid plates or cells in a 

 rudimentary state. The first kind is seen in cartilage ; here the walls 

 of the cell become increased in thickness and coalesce with each other, 

 mixing at the same time with the intercellular substance, while the 

 cavities or vacuolm remain distinct, but are rendered smaller by this 

 process of deposition. In the second kind, cells coalesce, but their 

 cavities or vacuolce run into each other : the tubules of some glands are 

 thus formed. Where the cells touch they coalesce, and the thin walls 

 dissolving, a single elongated cavity is formed. 



Another mode is, the cells may be aggregated, like a bunch of 

 raisins, and the parts in contact with each other disappear, so con- 

 stituting a multilocular cavity : examples of this are seen in the 

 racemose glands. Schwaun conjectures another mode of coalescence. 

 From cells formed as usual, processes sprout out ; but this change 

 takes place at the expense of the cell-membrane itself, and when 

 it has gone on to some extent, we have the appearance of a net- 

 work formed. Capillary vessels are in this way formed, as shown at 

 fig. 3 b and fig. 6, Plate XII. Cells, we thus perceive, coalesce to 

 form tissues, when they have not attained their full growth as such ; 

 or when they have been fully formed they become flattened, and assume 

 the solid form. Deposits of matter may take place from the cyto- 

 blastema with similar adjoining substance, constituting a delicate mem- 

 brane, with here and there nuclei, as in the capsule of the lens, the 

 membrane of the aqueous humour of the eye, or sheath of the primi- 

 tive fasciculus of muscle ; or the cells may coalesce in the linear series, 

 to form fibre. 



Development of Complicated Cells. Here the nucleated cell is sur- 

 rounded by a deposit, and that again surrounded so as to constitute a 

 membrane; so that the nucleated cell may be looked upon as the 

 nucleus to the cell so formed. Sometimes the nucleus undergoes im- 

 portant changes in the development of tissues, as well as the cell it- 

 self. In some cases, where the cells have joined in the linear series, 

 the nucleus becomes oval, elongated, so that the nucleus of one cell 

 tends to meet the nucleus of another cell ; they subsequently coalesce, 

 and thus fibre is formed. That so-constituted filament differs from 

 that formed by the coalescence of the cells themselves, which is acted 

 upon by nitric acid, whilst that formed from the nucleus resists it. 

 The nucleus may be on the exterior of the cell, and sometimes im- 

 bedded in the wall. As an example of the first, may be instanced the 

 nucleus of the pigment-cell of the eye, fig. 1 3, Plate XII. If, instead of 

 the fibre being flat, it is cylindrical, it is formed by the nucleus. When 



