1 84 



THE BIOLOGY OF THE FROG 



verted into a transparent substance which is discharged into 

 the central cavity (Biedermann), and it is probable that they 

 represent a stage in the formation of mucus. Numerous 

 transitional stages between these two varieties of epithelium 



occur, and it is quite certain 

 that the differences are due 

 to the age of the glands, and 

 their different states of secre- 

 tion. Changes in the form 

 of the cells, however, are pro- 

 duced to a certain extent by 

 the contraction of the gland. 

 Outside of the epithelium 

 is a muscular coat composed 

 of smooth muscle cells which 

 lie in a meridional direction. 

 The outermost coat of the 

 gland is formed by a layer 

 of fibrous connective tissue. 

 The function of the muscle 

 cells is the expulsion of the 

 secretion of the gland. The 

 glands of the skin are in con- 

 stant motion (Ascherson, En- 

 gelmann), as may be seen by 

 an examination of the glands 

 in the web of the foot. They 

 change not only in size, but 

 also in form, being now 





FUj. 47. — Cross section of the skin 

 of the frog. D, dermis or cutis ; E, 

 epidermis ; b.v, blood vessel ; c.gl, 

 cutaneous gland cut through the 

 center; c.gl' t the same from one 

 side ; d, duct of gland ; h.f, h.f\ 

 h.f", horizontal fibers of connec- 

 tive tissue ; h.l, outer or horny layer 

 of the epidermis ; m.l, Malpighian 

 layer of the epidermis ; pg, pigment 

 cells. (After Howes.) 



rounded and now wrinkled 

 and angular. Contraction may be caused by stimulation of 

 the skin with irritant solutions or by the electric current. 

 The poison glands are larger and less abundant than the 



