THE SKIN AND ITS APPENDAGES. 



28l 



hairs. Coincidently with the growth of the secondary shaft the old 

 dead hair becomes shifted towards the surface, loosened, and finally 

 entirely displaced. 



The development of the sebaceous glands starts as an out- 

 growth from the external root-sheath of the hair-follicle, from 

 which knob-like projections extend laterally ; these are at first solid 

 flask-shaped processes, the central cells of which become filled with 

 fat-particles. This fatty metamorphosis affects all the cells occupy- 

 ing the axis of the developing gland as far as the root-sheath ; after 

 a time the latter structure is penetrated and the degenerated fatty 

 cell-mass discharged into the hair-follicle as the first sebaceous 

 secretion. From the original tubular projection secondary com- 

 partments are produced by a repetition of the processes of budding 

 and subsequent hollowing out until the entire complement of saccules 

 has been formed. After the disintegration of the central cells, the 

 peripheral elements undergo similar change. 



The development of the sweat-glands follows closely that of 

 the hairs and the sebaceous follicles ; as in these, so here, the first 

 stage consists in the ingrowth, during the fifth month, of a solid 

 epithelial club-shaped process from the stratum mucosum into 

 the primitive corium. About the seventh month a lumen appears 

 within the tubular mass, an exit, 



however, for some time being FlG - 3 l6 - 



still wanting ; subsequently the 

 obstructing epidermal layers are 

 broken through. Somewhat 

 before the appearance of the lu- 

 men the extremity of the cylin- 

 der undergoes increased growth, 

 resulting in the thickening and 

 convolution of the tube which 

 represents the future coiled 

 division of the gland ; the full 

 expression of the characteristic 

 convoluted arrangement, how- 

 ever, is not attained until shortly 

 before birth. The muscular 

 tissue of the secretory tubes, 

 situated between the basement- 

 membrane and the lining epithe- 

 lium, is present before the close of the ninth month ; its origin is 

 as remarkable as its position, since the muscle-cells are derived 

 from the elements of the adjacent ectoderm. The basement-mem- 

 brane and the fibrous sheath are contributions from the mesoderm. 



. 



Section of skin of human foetus, showing devel- 

 oping sweat-glands. The latter grow as epithelial 

 cylinders from the stratum Malpighii of the epi- 

 dermis into the underlying corium ; the character- 

 istic coil appears later. 



