54 



GENERAL SKETCH OF THE CELL 



the reverse is the case in plants, where the cell-walls play a very 

 important role. In the latter the wall sometimes attains a great 

 thickness, usually displays a distinct stratification, and often has a 

 complex sculpture. Such massive walls very rarely occur in the 

 case of animal tissues, though the intercellular matrix of cartilage 

 and bone is to a certain extent analogous to them, and the thick and 

 often highly sculptured envelopes of some kinds of eggs and of 

 various Protozoa may be placed in the same category. 



It is open to question whether any cells are entirely devoid of an 

 enclosing envelope; for even in such "naked" cells as leucocytes, 

 rhizopods, or membraneless eggs, the boundary of the cell is usually 

 formed by a more resistant layer of protoplasm or " pellicle " (BiitschU) 

 which may be so marked as to simulate a true membrane, as is the 

 case, for example, in the red blood-corpuscles (Ranvier, Waldeyer, 

 etc.). Such pellicles probably differ from true membranes only in 

 deo-ree ; but it is still an open question both in animals and in plants, 

 how far true membranes arise by direct transformation of the periph- 

 eral protoplasmic layer (the " Hautschicht " of botanists), and how 

 far as a secretion-product of the protoplasm. In the case of animal 

 cells, Leydig long since proposed ^ to distinguish between " cuticular " 

 membranes, formed as secretions and usually occurring only on the 

 free surfaces (as in epithelia), from " true membranes " arising by 

 direct transformation of the peripheral protoplasm. Later researches, 

 including those of Leydig himself, have thrown so much doubt on 

 this distinction that most later writers have used the term cuticular 

 in a purely topographical sense to denote membranes formed only 

 on one (the free) side of the cell,^ leaving open the question of origin. 

 The formation and growth of the cell-wall have been far more thor- 

 oughly studied in plants than in animals, yet even here opinion is 

 stifl divided. Most recent researches tend to sustain the early view 

 of Nageli that the cell-wall is in general a secretion-product, though 

 there are some cases in which a direct transformation of protoplasm 

 into membrane-stuff seems to occur.^ In the division of plant-cells 

 the daughter-cells are in almost all cases cut apart by a cell-plate 

 which arises in the protoplasm of the mother-cell as a transverse 

 series of thickenings of the spindle-fibres in the equatorial region 

 (Fig. 34). This fact, long regarded by Strasburger and others as 

 a proof of the direct origin of the membrane from the protoplasmic 

 substance, is shown by Strasburger's latest work ('98) to be open 

 to a quite different interpretation, the actual wall being formed by 

 a splitting of the cell-plate into two layers between which the wall 

 appears as a secretion-product. Almost all observers further are 

 ao-reed that the formation of new membranes on naked masses of 

 1 Cf. '85, p. 12. ^ Cf.Q. Hertwig, '93. * Cf. Strasburger, '98. 



