398 THE EXOSKELETON. 



the cuticle, and appears first at the inner end of the duct and 

 thence extends outwards (Ranvier, No. 311). In the sebaceous 

 glands the first secretion is formed by a fatty modification of the 

 whole of the central cells of the gland. 



The muscular layer of the secreting part of the sweat-glands 

 is formed, according to Ranvier (No. 311), from a modification 

 of the deeper layer of the epidermic cells. 



The Mammary Glands arise in essentially the same man- 

 ner as the other glands of the skin 1 . The glands of each side 

 are formed as a solid bud of the mucous layer of the epidermis. 

 From this bud processes sprout out, each of which gives rise to 

 one of the numerous glands of which the whole organ is formed. 

 Two very distinct types in the relation of the ducts of the 

 glands to the nipple are found (Gegenbaur, No. 313). 



BIBLIOGRAPHY OF EPIDERMIS. 

 General. 



(304) T. H. Huxley. "Tegumentary organs." Tocld's Cyclopedia of Anat. 

 and Physiol. 



(305) P. Z. Unna. " Histol. u. Entwick. d. Oberhaut." Archiv f. mikr. Anat. 

 Vol. xv. 1876. FzVfcalso Kolliker (No. 298). 



Scales of tlic Pisces. 



(306) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d. 

 Zahne d. Selachier." Jcnaische Zeitschrift, Vol. vin. 1874. 



(307) O. Hertwig. "Ueber d. Hautskelet d. Fische." Morphol. Jahrbmh, 

 Vol. II. 1876. (Siluroiden u. Acipenseridre.) 



(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypte- 

 rus)." Morph. Jahrlntch, Vol. V. 1879. 



Feathers, 



(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873. 



(310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit. f. -cuss. Zoo/., Vol. 

 xxx. 1878. 



Sweat-glands. 



(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes 

 Jfemitts, Dec. 29, 1879. 



1 For a very different view on this subject vide Creighton (No. 312). 



