SECT. 75.] 



SEBACEOUS GLANDS. 



l 39 



follicles, inasmuch as they appear contemporaneously with the 

 commencing hairs, or, a short time after, as outgrowths of the outer 

 root sheath (fig. 52). These are, at first, from o'Oi'" to 0"03'" in dia- 

 meter, and from 001'" to o - oi6'" in thickness, but soon become 

 larger and pyriform, or flask-shaped. Fat is then developed in the 

 inner cells, till at last the fat-cells extend as far as the canal of the 

 hair-follicle. The gland and its contents are now formed, and a 

 multiplication of the cells at the bottom of the gland, or in the 

 gland-vesicle, is all that is requisite to push the sebaceous cells of 

 the duct into the hair-follicle, and fully establish the secretion. 

 Accordingly, the sebaceous, like the sudoriparous glands, are, at 

 first, solid outgrowths of the Malpighian layer of the skin, which 

 subsequently acquire external orifices, and the first sebaceous 

 matter of the skin is formed by a metamorphosis of the inner cells 

 of the glandular rudiments; whilst the space which these cells 

 occupy becomes the cavity of the gland, which, however, never 

 appears hollow, but is kept filled by the continual advance of the 

 growing cells. 



The above-described development of the sebaceous follicles takes 

 place rather rapidly. In the fifth mouth, the secretion commences 

 in many places ; and in the sixth, it is everywhere fully established. 

 It is further to be remarked, that besides the original glands, 

 which are attached singly or in pairs to a hair-follicle, in the sixth 

 month new rudiments arise, which generally have a deeper position ; 

 and after going through the above-described process of develop- 

 ment, assume the form of secreting glands. 



In the further development of the sebaceous glands, the origin- 

 ally simple gland-tube puts forth 

 solid buds by the growth of its ex- 

 ternal fatless cells, which buds are 

 gradually transformed into glandular 

 vesicles in exactly the same manner 

 as the first rudiments. By repeated 

 budding of the primitive or secon- 

 dary glandular vesicles, the larger 

 clusters are formed, and from these, 

 finally, the most complex that ap- 

 pear. In the foetus of the seventh 

 month, the majority of the glands 

 are still simple pedunculated tubes 

 of 004'" to 006'" in length, and 

 002'" to o - c>3'" in breadth, which 



Fig. 62. 



Development of a sebaceous gland in a six 

 months' fetus, a. Hair; b. inner root-sheath ; 

 c. outer root-sheath ; d. rudiment of a gland, 

 with fat developed in the central cells. Mag- 

 nified '250 times. 



