' CELLS. 69 



Donders has recently promulgated the view that it is only the cell- 

 contents which are contractile, not the cell-membranes. xVlthough it 

 must be granted, that it is difficult, in the cases in question, to decide 

 what part of the cell contracts, yet it seems very hazardous to endeavor 

 to refer the movements of the cilia in plants and animals, in free and 

 combined cells, to indemonstrable contents in these cilia communicating 

 with the cell. In the cells of the Planarije and in the Protozoa, any 

 one who has actually seen the movements will hardly refer them to 

 anything but the cell-membranes. In the transversely striated muscular 

 fibres on the other hand, it is evidently the fibrillaa or the contents 

 which are contractile, and the sarcolemma, as an elastic yielding body, 

 only moves with them : the same appears to hold good with the muscular 

 fibre-cells, in which a special membrane cannot bo demonstrated. 



§ 18. Metamorphoses of Cells — Kinds of Cells. — The destination of 

 the cells which are found at an early period in the organism is very 

 various. A very considerable portion of them remain but for a short 

 time in their primitive condition, and subsequently coalesce with others 

 to form the higher elementary parts. Another portion, while they enter 

 into no such combinations, change more or less their previous nature ; 

 as the horny plates of the epidermis and nails. Many cells, lastly, 

 never become metamorphosed at all, but remain as cells, until sooner 

 or later, often not before the decay of the organism, they disappear 

 accidentally or typically, as the epithelia, glandular parenchymata, &c. 



The permanent cells may be most conveniently arranged under the 

 following heads : — 



1. True cells, which have in no essential respect altered their cellular 

 nature. These occur in the epidermis [stratum Malpigldi) and the 

 epithelia; in the blood, chyle, lymph; in the glandular secretions, in 

 the fatty tissue, in the gray nervous substance, the red bone-medulla ; in 

 the glands (liver, spleen, suprarenal capsules, closed glandular follicles), 

 and the cartilages. According to their form, these cells may be divided 

 into round, discoid, cylindrical, conical with cilia, and stellate; accord- 

 ing to their contents, they may be distinguished as containing fat, protein 

 or serum, hsematin, bilin, pepsin, mucus, or pigment ; and as to their 

 modes of occurrence, some are either isolated in fluids or in solid tis- 

 sues, others are united into a simple cellular parenchyma, while others 

 are conjoined by an intercellular substance of one kind or another. 



It is certainly the membrane which contracts in these cases, for it pushes out processes 

 which are only subsequently filled by the granular contents. 



In the lower plants (Alga) the occurrence of contractile processes in the shape of cilia is 

 universal, and the contractility of the cell substance in the zoospores of Volvox is evinced 

 by the occurrence of a rhythmically contracting space in them. [See Busk on Volvox globator, 

 '' Quarterly Journ. of Micr. Sc," No. 2, 1853.)— Tks.] 



