ON WANDERING CELLS IN ECHINODERMS. 93 
Gaskell (No. 32, p. 39) regards it likely that pigment is got 
rid of in Ammoceetes, &c., by “ excretion into the skin.” 
J. H. List (No. 48) describes the formation of pigment 
granules in red blood-corpuscles, and their travel to the surface 
within wander-cells ; moreover he definitely considers that the 
pigment is to be regarded as a “ Zerfallsproduct” or an 
‘“‘ Exkretionsproduct ;” he also gives many references. 
Leydig (No. 45), in a most interesting paper, calls attention 
to the different methods of external pigmentation ; in snakes, 
&c., he shows that white and whity-yellow patches owe their 
colour to the presence of stellate cells containing granular 
concretions, which concretions consist chiefly of a uric acid 
holding substance (guanin), probably crystalline in form, with 
a proteid base. Such cells occur both in the cutis and amongst 
the epidermal cells, similarly to the cells containing dark pig- 
ment-granules. The following (from Section IX, p. 260) is 
worth quoting :—‘“ Waren in den Vorausgegangenen Hautfar- 
bungen, so bald sich um wirkliche Farbkorper handelt, diese 
stets innerhalb der Gewebe des Integuments abgelagert, so 
gibt es endlich eine ganze Anzahl von Farbungen, welche 
Erzeugnisse von Hautsekreten sind, nach aussen gelangen und 
daher abwischbar werden.” 
Besides amphibians and reptiles he has demonstrated the 
presence of urate compounds in the skin of a mammal (Chry- 
sochloris), and remarks that it would be well worth while look- 
ing for such chemical bodies in feathers, hair, &c., of higher 
Vertebrates. In Vertebrates there is another organ—the iris— 
where guanin compounds (“ guaninhaltige pigment”’) exist. In 
Invertebrates he has shown pigmentation to be due to a uric 
acid holding body in Asellus, Syrphus, certain slugs, and 
(quoting Eisig) in Capitellide. It is interesting to note, in 
relation to Dr. Gaskell’s ideas on the meaning of the pigment 
about the brain, that Leydig states (p. 256) that he has not 
observed any contractile power in the pigment-cells of the 
dura mater in frogs. 
Recently Kodis (No. 42) has opposed the view that the 
leucocytes in the epithelium have arrived there from subjacent 
