1902 Hertwig O

 
Die Entwicklung der Haut und ihrer Nebenorgane. Lehrbuch der Entwicklungsgeschichte des Menschen und der Wirbeltiere. Jena: Fischer, 1902;7:547–566 (Fig. 4).

View Figure  		Fig. 4  A, Development of the papilla on the skin surface. B,C,D, Three different phases of hair development in human embryos. td, Sebaceous gland.
 

Message

 
Sebaceous glands develop either directly from the epidermis or from solid thickenings of the outer sheath of the follicle. These bottle-shaped thickenings then produce single lateral sprouts that expand at their periphery.
 
Hertwig described what he interpreted to be the development of the sebaceous gland in its entirety, beginning with a protuberance of the embryonal follicle, progression of that protuberance into a mantle, and eventuation of a mantle into sebaceous cells that in aggregate develop sac-like ends, i.e., lobules.
 

Critique

 
Sebaceous glands do not develop directly from the epidermis.
 
The mantle is continuous with the follicle at the junction of the infundibulum and outer sheath at the isthmus, the precise site of the intermediate bulge in an embryo.
 
Hertwig did not realize that the single lateral sprouts which he observed were actually the mantle described by Pinkus as seen in longitudinal section.
 
Hertwig seems to have been the first to appreciate the fact that mantles follow formation of sebaceous lobules in embryonic life and later give rise to sebaceous lobules anew.