MAYER: DEVELOPMENT OF WING SCALES. 225 
wing, and it is remarkable that only a single leucocyte enters each of 
these, The fact that the leucocyte always disintegrates, is also signifi- 
cant, for it suggests that the hemolymph within the scale is not ina 
normal or healthy condition.. Furthermore, the fact that the lymph of 
the scale becomes turbid and of a yellow-ochre color, may perhaps be 
attributable to its being shut up within the scale, and thus cut off from 
the possibility of renewal. At first | thought that the entrance of the 
leucocytes into these scales might be related to the fact that the scales 
which lie upon the nervures and at the edges of the wings are always 
the last to acquire their mature colors ; but this is not so, for in Callo- 
samia promethea the scales that are found upon the nervures are not 
any larger than those that lie between them, all being far too small 
to admit the entrance of leucocytes; and yet in this case also the 
scales upon the nervures and at the edges of the wing are the last 
to acquire their mature colors. The entrance of the leucocyte seems 
therefore to have nothing whatever to do with the pigmentation of the 
scale. I believe it is due merely to the fact that the scales upon the 
nervures and at the edges of the wings in Danais plexippus are large 
enough to admit leucocytes. 
But to return to the general discussion of the development of the pig- 
ment. After the wings have remained in the ochre-yellow stage for 
about twenty-four hours, the mature colors begin to show themselves. 
These mature colors always appear first within scales which are situated 
between the nervures. They are faint at the beginning, but gradually 
increase in intensity. For example, if a scale be destined to become 
black, it first becomes pale grayish brown, and this color gradually 
deepens into black. This pigment is no doubt derived from the 
hemolymph within the scale, for the simple reason that there is 
nothing but heomolymph within the scale at the time when it first 
appears. It is probably produced by chemical processes that are some- 
what analogous to the clotting of the blood, for the pigment is found to 
be sublimed over all the surfaces of the cavity of the scale, as is shown 
in black in Plate 6, Fig. 44. 
It was first pointed out by Burmeister (’78), that the layer of pig- 
ment is especially thick upon the upper surface of the scale (i. e. the 
surface which is away from the wing membrane). 
