SECONDARY DEPOSITS. [ 686 ] SECONDARY DEPOSITS. 



tlie gland ; this is derived either from the 

 hair-follicle or the cutis, according to the 

 situation ©f the gland. It is lined by layers 

 of roundish or polygonal, epidermic or 

 epithelial cells, the outermost of which 

 are closely connected, so as to form one or 

 more membranous layers, and contain few 

 or no globules of fat ; whilst the inner ones 

 are larger, and almost filled with these glo- 

 bules. 



The development of the sebaceous glands 

 commences at the end of the fourth or in 

 the fifth mouth. The glands at first consist 

 of solid depressions or outgrowths of the 

 rete mucosum of the skin, or the inner 

 root-sheath of the hairs ; the inner cells 

 then become filled with fat, loosened, and 

 are finally evacuated through that part of 

 the immature gland which in its subsequent 

 development forms the duct. 



BiBL. KoUiker, Mik. An. i. 180 ; Biesia- 

 decki, Strieker's Hist. 



S E C O N D A R Y D E P S I T S OB 

 LAYERS OF Vegetable Cells. — The 

 structures known by this name are spoken 

 of under Cells, in a general point of view, 

 and in detail under Pitted and Spiral 

 Structtjees. a few remarks maybe given 

 here, connecting the phenomena included 

 under the last two heads. 



It is well known that the original or 

 2yritnarif cell-wall, the layer of cellulose by 

 which the cell first becomes really consti- 

 tuted as a closed membranous sac, is, so far 

 as our present instruments enable us to 

 judge, devoid of detailed structure : it is a 

 homogeneous pellicle. This has a power of 

 extension by interstitial nutrition, which 

 leaves no traces in the perfect membrane, 

 enabling the cell to increase in size. But 

 the increase in solidity is effected by a 

 different process, leaving distinct evidences 

 of its occurrence, namely by an application 

 of successive thin layers of cellulose mem- 

 brane, more or less completely all over the 

 inside of the primary membrane, giving the 

 cell-wall a laminated character, either evi- 

 dent in the natural condition, or capable of 

 being demonstrated by the aid of macera- 

 tion or corrosive applications. 



No cell which is to form part of a ])erma- 

 neut tissut; remains long without receiving 

 secondary layers upon its walls. In certain 

 cases the wall (sxliibits in its natural state 

 merely the laminated htructure, withciut 

 any markings (PI. 47. fig. 24) ; but in the 

 majority of cases, where the secondary de- 

 posits are considerable, these layers exhibit 



markings of very peculiar characters. As a 

 general rule, the layers present themselves 

 under two different types, according to the 

 extent to which they cover the primary 

 membrane. In one case they are applied as 

 a general layer over the wall, absent merely 

 at dot-like or slit-like points, where they 

 leave the primary wall uncovered, and thus 

 give rise to a pitted condition as seen from 

 the inside of the cell. Successive layers 

 lea^•iug the same spots bare, the pits become 

 gradually deeper, and form canals running 

 through the thick cell-wall to the primary 

 membrane (see Pitted Structures) (PI. 

 47. fig. 23). 



Another curious condition of the secon- 

 dary deposits has been pointed out by Hartig 

 and Mohl, where large patches or spots upon 

 the cell-wall, especially at its base, are left 

 bare by the thicker secondary layers, and 

 become coated with a thin layer perforated 

 by minute orifices, as if riddled with holes, 

 or reticulated ; this is called a sieve-tube 

 or dathrate tissue. 



In the other case, the secondary deposits 

 are more sparing in quantity, and are ap- 

 plied over lines forming a definite pattern 

 upon the primary membrane, in which a 

 spiral course in the direction of the long 

 axis of the cell is more or less evident ; 

 infinite modifications of this type occm*, 

 which are treated of under the head of 

 Spiral Structures (PI. 48. figs. 7, 9). 



In certain less common cases we find the 

 earlier secondary layers exhibiting the pitted 

 character, while others later-formed produce 

 spiral-fibrous thickening, as in TaxHS, the 

 lime, and other cases (see Pitted Struc- 

 tures, PL 48. figs. 4, 13, 19 i). 



The last-mentioned cases point to a rela- 

 tion between the spiral-fibrous and the 

 pitted layers, which appears really to exist, 

 for in a great number of cases it is possible 

 to distinguish a spiral structure in the 

 membranous layers of pitted cells, or even of 

 cells where the layers of thickening luerelv 

 exhibit the laminated structure without an}"- 

 pits or fibrous markings. Thus, in the liber- 

 cells of the Apocynace* (PI. 48. fig. 30), 

 the thickened walls appear under a low 

 power homogeneous, while under higher 

 magnifying-power, and especially bj^ the 

 help of acids, we may detect an evident 

 though delicate spiral structure. The action 

 of acids reveals a similar. sjiiral arrangement 

 of the constituent molecules, in the cotton 

 hair (PI. 28. tig. 1), and in most liber-cuUs 

 (figs. 2, 5, 25), in many wood-cells, as of 



