CELL. 



[ 138 ] 



CELL, 



Occasionally the membrane lias a granular 

 appearance, arising from projections de- 



Kendent on granules lying on the inside. 

 To structure can be detected in the cell- 

 membrane itself. 



The membranous cell generally contains 

 a liquid or semifluid protoplasm, the consis- 

 tence of which varies ; in this float, or are 

 suspended, molecules, granules, globules, or 

 other very minute cells. But modern ob- 

 servations indicate the existence of a net- 

 work of filaments, traversing the substance 

 of the protoplasm. In most cells, we also 

 find one or more of those bodies which are 

 termed nuclei, often attached to some jiart 

 of the cell-wall ; in these again, a network 

 of fibres is stated to occur. The nuclei con- 

 tain nucleoli. The cell-contents likewise in- 

 clude, in particular structures, products of 

 secretion— matters separated by the cells 

 from the circulating fluid, as in the cells of 

 the renal epithelium, &c., — also crystals, 

 pigment, &c. 



The /or??JS presented by animal cells are 

 not so varied, or generally so geometrical, 

 as those occurring in the cells of vegetables. 



In regard to size, the largest are the yelk- 

 cells of ova, especially of Birds and Reptiles, 

 and of some animals consisting of a single 

 cell, as certain of those curious organisms 

 the Greyarince. 



The nuclei are usually spherical or lenti- 

 cular, non-contractile, transparent, and co- 

 lourless or yellowish. They are sometimes 

 solid or homogeneous ; at others they are 

 vesicles, with a very delicate membrane. 

 They sometimes contain, exclusively of the 

 nucleolus, a transparent colourless or yel- 

 lowish liquid, in which water and acetic 

 acid produce a precipitate of gramdes re- 

 sembling those existing in the cell-contents ; 

 hence in the ordinary manner of examining 

 them they seldom present their natural 

 transparency. The nuclei seem to be se- 

 creted or formed by the protoplasm, and 

 are regulators of the vital plienomena. 



The nucleoli are roimded, well defined, 

 very minute, sometimes immeasurable. 



Chemically, the cell-membrane ordinarily 

 consists of a proteine compound; it is mostly 

 dissolved, or rendered so transparent as to 

 become invisible, by acetic acid and solution 

 of potash ; thus differing in a striking man- 

 ner from that of vegetable cells. Cell-mem- 

 branes composed of cellulose occur in some 

 animals, as in the Tunicata &c. ; it is de- 

 tected here, as in plants, by the action of 

 iodine and sulphuric acid. The nucleoli 



consist also of a proteine compound ; they 

 are soluble in potash,' but not in acetic acid. 

 The action of potash distinguishes them 

 from globules of fat. The carmine-am- 

 monia solution has a much more rapid and 

 powerful dyeing action upon the protoplasm 

 and the nuclei than upon the cell-walls. 



It must be remarked that the appearances 

 interpreted to be nuclei and nucleoli are 

 frequently not respectively identical in 

 kind; the nuclei are sometimes homogene- 

 ous, at others true cells; sometimes they 

 relate to the formation of the cell ; at others 

 they are young secondary cells, vacuoles, 

 &c. ; the same applies to the nucleoli. These 

 important points have not hitherto received 

 sufficient attention. 



Cells, or rather their protoplasms, are en- 

 dowed with peculiar vital forces, by which 

 they are capable of free movement, absorp- 

 tion, and the elaboration of the absorbed 

 matter; of growth, reproduction, and of 

 secretion. The entire organism of the higher 

 and most of the lower animals consists, at 

 a certain period of life, of cells, but some- 

 times of protoplasm alone. 



Formation of Cells. — Cells are formed in 

 two ways : either from a blastema, proto- 

 plasm, or formative substance existing with- 

 out, or contained within, other cells. The 

 protoplasm is a semifluid substance, consist- 

 ing of proteine or albuminous matter, fatty 

 matter and salts. 



The formation of cells in a free blastema 

 is not a general process ; in fact, its occur- 

 rence is now mostly denied. The only in- 



Fig. 104. 



Magnified 350 diameters. 



Contents of a Jliilpighian body from the spleen of an 

 01. a, small, b, larger cells; c, free nuclei. 



stances of its supposed occurrence in man 

 and the higher anim;^ls were in the forma- 

 tion of the chyle and lymph corpuscles, the 

 cells of certain glandular secretions (seminal 

 cells, ova), and glandular organs (the closed 

 follicles of the intestine, the lymphatic 

 glands, the splenic corpuscles with the 

 splenic pulp, and the thymus) ; lastly, of 

 the cellular elements in the impregnated 

 uterus, in the corpus luteum, the marrow of 

 foetal bones, and in the soft ossifying bias- 



