1 84 



THE BIOLOGY OF THE FROG 



CHAP. 



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 



FIG 47. -Cross section of the skin ^^ ^^ ^^^ ^^-^ ^^^ -^ ^^^_ 



of the frog. D, dermis or cutis ; Ji, c 



epidermis ; i.v, blood vessel ; c.£-/, stant motion (Ascherson, En- 

 cutaneous gland cut through the ggij^^nn) , as may be seen by 

 center; c.£-l', the same from one «= ; . r i i j 



side; d, duct of gland; A./, k./', an examuiation of the glands 



k./", horizontal fibers of connec- ^^ |_jjg ^gl^ Qf ^^g foot. They 



tive tissue; /^.Z, outer or horny layer • „ u.,4. 



of the epidermis; ;«./, Malpighian change not only m Slze, but 



layer of the epidermis; /^.pigment ^IsO in form, being nOW 



cells. (After Ho^ves.) 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 



