SEBACEOUS FOLLICLES. [ 570 ] SECONDARY DEPOSITS. 



areolar tissue, forming a more or less thick 

 membrane in proportion to the size of the 

 gland ; this is derived either from the hair- 

 Fig. 643. 



Development of the sebaceous follicles in a six-months' 

 fetus, a, hair ; b, inner root-sheath ; c, outer root- 

 sheath ; d, rudimentary follicle. 



Magnified 250 diameters. 



Fig. 644. 



a, b t c, d, as above, but in a more advanced stage. 

 Magnified 250 diameters. 



follicle, or the cutis, according to the situa- 

 tion of 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 mem- 

 branous layers, and contain few or no glo- 

 bules of fat ; whilst the inner ones are larger, 

 and almost filled with these globules. 



The development of the sebaceous glands 



commences at the end of the fourth or in 

 the fifth month. 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 de- 

 velopment forms the duct. 



BIBL. Kolliker, Mikroskop, Anat. i. 180, 

 and the Bibl. of that article. 



SECONDARY DEPOSITS OR LAYERS, 

 OF VEGETABLE CELLS. The structures 

 known by this name are spoken of under 

 the head of CELLS, in a general point of 

 view, and in detail under PITTED and SPIRAL 

 STRUCTURES. A few remarks may be given 

 here, connecting the phenomena included 

 under the last two heads. 



It is well known that the original or pri- 

 mary cell-wall, the layer of cellulose by 

 which the cell first becomes really constituted 

 as a closed membranous sac, is, so far as our 

 present instruments enable us to judge, de- 

 void of detailed structure ; it is a homoge- 

 neous 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 membrane, more or less 

 completely all over the inside of the primary 

 membrane, giving the cell-wall a laminated 

 character, either evident in the natural con- 

 dition, or capable of being demonstrated by 

 the aid of maceration or corrosive application s. 



No cell which is to form part of a perma- 

 nent tissue remains long without receiving 

 secondary layers upon its walls. In certain 

 cases the wall exhibits in its natural state 

 merely the laminated structure, without any 

 markings (PI. 38. fig. 24); but in the majority 

 of cases, where the secondary deposits 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 leaving the 

 same spots bare, the pits become gradually 

 deeper, and form canals running through 



