PIGMENT CHANGES IN ANUEAN LARVAE 127 



transplanted thyroid glands from animals dead over twelve hours, 

 this point has been well brought out. 



A word regarding the various types of pigment cells found in 

 the frog skin may serve in some measure to clarify the discussion 

 to follow. There are several varieties of pigment cells which nor- 

 mally occur in the skin of anurans, and of these there may be 

 distinguished: the melanophores or black pigment cells, leuco- 

 phores or interference cells, xanthophores, and xantholeucophores 

 or golden pigment cells. Some species of frogs have red pigment. 



The melanophores are regarded by the majority of investi- 

 gators as fixed stellate cells with many irregular branching proc- 

 esses. The pigment is in the form of numerous minute brown or 

 black granules of melanin scattered in the cytoplasm. This 

 type of chromatophore is most abundant on the dorsal surface of 

 the body, and occurs in dense masses in the black spots found 

 scattered over the back and sides of the animal. In sections 

 through the skin fig. 3 B) they are found mostly in the superficial 

 layer of the corium just below the epidermis. A somewhat differ- 

 ent type of chromatophore is found in the epidermis; it consists 

 of a cell body with two or more simple processes. Such cells 

 usually lie singly in the epidermis (especially in lightly pigmented 

 individuals) , though they may occur in darker animals in suffi- 

 •cient abundance to give the appearance of a reticulum. This 

 type of melanophore is said by Hooker ('14) not to be contractile, 

 though the writer is inclined to question this assertion, especially 

 as regards conditions in Rana catesbeiana. On the ventral" 

 surface of the tadpole and frog body, these two types of pigment 

 cells are almost absent over considerable areas. The subepider- 

 mal melanophores are innervated by motor fibers proceeding 

 along both sympathetic and spinal paths, but, as Laurens ('15) 

 has shown, this type of pigment cell may also respond to direct 

 stimulation without the intermediation of the nervous system. 

 As shown in figure 35, the subepidermal melanophores present, 

 when contracted, the appearance of small rounded densely 

 staining masses. 



According to Smith ( '20) , the xantholeucophores of the anuran 

 larva form two well-defined layers over the dorsal portion of the 



