48 



GENERAL ZOOLOGY 



subcutaneous tissue of the breast. In the secretion of milk, the 

 free end of the gland cell ruptures and discharges fatty droplets 

 along with cell substance. The empty cells then fill up. Few 

 gland cells are cast off. The ceruminous (wax) glands of the 

 external auditory meatus are also modified sweat glands. 



Nails and Claws. — A cross section of a finger through the 

 nail shows that the nail is composed of a layer of cornified cells 

 overlying a stratum germinativum (Fig. 23). The corium of the 

 nail bed consists of fibrous and elastic connective tissue running 

 vertically from the periosteum of the bone (phalanx of the finger) 

 to the stratum germinativum, and also of connective tissue 

 fibers running the length of the finger. The cornified layer of the 

 nail represents modified epidermal cells. In the embryo this 



A B 



Fig. 23. — The nail of a human fetus of 10 weeks. A, dorsal view; B, longitu- 

 dinal section; b, nail bed; e, eponychium; h, hyponychium; 1, lunula; n, nail; s, 

 nail sulcus. (After Kollman.) 



layer is covered by the eponychium, which in the adult is reduced 

 to the thin layer at the base of the nail, continuous with stratum 

 corneum of the contiguous skin. The nail is produced from the 

 stratum germinativum at the root of the nail and as far forward 

 as the boundary of the lunula, the crescentic white area at the 

 base of the nail. Claws are similar modifications of the epidermal 

 layer. 



Hair and Feathers. — In the embryo, the development of a hair 

 begins as a solid downgrowth of the basal layer of the epidermis, 

 terminating in a bulb, which becomes indented from below. 

 The indented region is occupied by cells from the corium, form- 

 ing the papilla which in later stages is provided with blood vessels. 

 The central part of the column above the papilla separates from 

 the peripheral cells to form a core, from which the shaft of the 

 hair develops (Fig. 24). The peripheral cells of the hair column 

 form the sheath of the hair. The sebaceous glands develop from 



