182 THE BIOLOGY OF THE FROG CHAP. 



* 



The stratum compactum is mainly composed of a dense 

 layer of connective tissue, whose fibers run in a wavy course 

 parallel to the surface of the skin. At intervals this layer 

 is crossed by vertical strands, which often extend through 

 the stratum spongiosum into the epidermis. In addition to 

 fibrous connective tissue, these strands frequently contain 

 smooth muscle fibers, elastic fibers, nerves, and blood ves- 

 sels. It is probably due to the contraction of these muscle 

 fibers that the papillation of the skin is produced after cer- 

 tain conditions of stimulation. The fibers may also aid in 

 squeezing out the secretion of the cutaneous glands. 



The subcutaneous connective tissue forms a loose layer 



j 



beneath the stratum compactum and a second very thin 

 layer next to the muscles, the two layers being separated by 

 large lymph spaces except in the septa, where they become 

 continuous. The outer of the two layers is very vascular 

 and contains numerous stellate cells, within which are nu- 

 merous grayish white pigment granules. These cells are 

 especially abundant on the ventral side of the body, where 

 they produce the white coloration characteristic of that 

 region. 



Glands of the Skin. - The skin of the frog, like that 

 of most of the Amphibia, is richly furnished with glands. 

 These glands are of the simple alveolar type, and lie mainly 

 in the stratum spongiosum of the corium. Only rarely, as 

 in the Lirge glands of the inner finger, do they extend into 

 the deeper portions of the skin. Typically the glands are 

 spherical or oval in form, and open to the surface through 

 a narrow neck which extends through the epidermis and 

 terminates in the triradiate opening of a so-called stoma 

 cell at its outer end. 



The skin glands of the frog have been studied by numer- 

 ous investigators, but there remain the widest differences 



