528 EMBRYOLOGY. 



Owing to the addition of new cells from the mucous membrane, both 

 from below and from the posterior margin, the nail-plate grows it 

 becomes thickened and increased in surface extent. It is now 

 pushed forward from behind over the bed of the nail, and at the 

 seventh month its free margin begins to project beyond the latter. 



With this the nail has acquired essentially the appearance and con- 

 dition which it has in the adult. In new-born infants it possesses a 

 margin which projects far over the ball of the finger, and which 

 because it was formed at an early embryonic period is both much 

 thinner and also narrower than the part formed later, which rests on 

 the bed of the nail. This margin is therefore detached soon after birth. 



(d) The Glands of the Skin. 



The glandular structures of the epidermis, which are established 

 by invagination, are of three kinds: sebaceous, sweat-, and milk- 

 glands. They all arise as proliferations of the mucous layer which 

 grow down as solid plugs into the derma, and then undergo further 

 development either according to the tubular or the alveolar type. 



The sweat-glands and the ear-wax glands are developed on the 

 tubular plan. They begin in the fifth month to penetrate from the 

 mucous membrane into the corium ; in the seventh month they 

 acquire a small lumen, take a winding course in consequence of 

 increased growth in length, and become coiled especially at their 

 deep ends, thereby giving rise to the first fundament of the 

 glomerulus. 



Sebaceous glands and milk-glands are alveolar structures. The former 

 are either developed directly from the- epidermis, as, for example, at 

 the edges of the lips, on the prepuce and on the glans penis, or they 

 are in close connection with the hairs, which is the ordinary condi- 

 tion. In the latter case they are formed as solid thickenings of the 

 outer sheath of the root of the hair near the orifice of the follicle, 

 even before the hairs are completely developed (fig. 292 C, D, td) at 

 first they have the form of a flask, then they send out a few lateral 

 buds, which develop club-shaped enlargements at their ends. The 

 glands acquire cavities by the fatty degeneration and disintegration 

 of the interior cells, which are eliminated as a secretion. 



The development of the milk-glands, which are more voluminous 

 organs entrusted with an important function and peculiar to the 

 class Mammalia, is of greater interest. Of the numerous works 

 that have appeared concerning them, the comparative-anatomical 

 investigations of GEGENBAUR especially have led to valuable results. 



