ON WANDERING CELLS IN ECHINODERMS. 103 
pigmentation is caused by wander-cells (“ carrier-cells ’’) 
passing their pigment on to the stratum Malpighii (vide 51). 
H. v. Planner (52) describes a case of congenital nevus in 
which there was considerable tumour formation as well as 
pigmentation. He figures pigment-bearing cells working 
their way up into and through the epidermis. Here also, 
then, there is evidence that a process of an excretory nature 
may take place. 
Why should pigment-cells travel towards the free surface ? 
It is difficult to fix upon any one determining cause, but it 
seems most probable that light has some guiding influence 
upon them. The movements which can be elicited in the 
pigment-cells of, e.g., frogs, and the chromatophores of Sepia, 
&c., show that light has some effect upon these structures ; 
moreover it is usual to find the dorsal surface of animals 
more deeply pigmented than are the ventral and more shaded 
ones. On account of the pigmentation of internal organs in 
some animals, and of animals living at great and lightless 
depths/, Kisig (27) denies that light is to be considered as a 
factor determining the production of pigment, though it may 
subsequently act upon or otherwise modify it. 
Leydig (45) considers that there is some connection between 
nerve-filaments and the processes of the pigment-cells in the 
skin; and, notwithstanding the opposition which has been 
raised to this view, it is difficult to understand how sym- 
metrical patterns could ever be produced without a bilaterally 
acting guide, such as the nervous system, although it is by no 
means clear how nervous interference could act. 
There seems to be some selective capacity on the part of the 
cells themselves, which causes them to steer for certain havens ; 
for instance, although pigment-cells occur abundantly in the 
somatic peritoneum and about the renal tubules in the newt, 
yet they do not wander through the epithelium of those 
tubules. In some cases the mechanical resistance offered by 
the tissues may be the real factor which prevents intrusion of 
wander-cells ; for instance, sections through the pineal eye and 
neighbouring structures in Lacerta show that, whilst there is 
