218 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
duced it; the nuclei being seen through a folded-over portion of the 
membrane. 
The strueture and arrangement of the formative cells, and of the 
undifferentiated hypodermis cells, are shown in Plate 2, Fig. 9. The 
body of the large formative cell (cl. frm.) lies wholly below the level of 
the hypodermis cells, and sends a protoplasmic process upward to form 
the scale (sg.). The nucleus of the formative cell is large, and spherical 
or ovoid in shape. At this stage it always exhibits a small highly 
refractive nucleolus situated near its centre. The chromatin consists of 
small curved and broken rods or filaments, and appears to be arranged in 
a spherical shell about the nucleolus as a centre. The scale (sq.) at this 
stage is a minute flattened chitinous bag filled with protoplasm. It. may 
be here noted that, as long as the scales remain full of protoplasm, they 
appear as transparent as glass, but when the protoplasm begins to shrink 
out of them, as it does later, and air takes its place, they become pure 
white. 
The hypodermis cells (/’drm.) have greatly changed in appearance 
since the stage represented in Figures 5 and 6. They are no longer 
sharply separated by well defined cell walls, but the protoplasm of 
adjacent cells has apparently become confluent. Nevertheless, each 
cell territory is quite well marked out by the peculiar arrangement of 
the finely granular contents of the cells. The region of the boundary 
between cells is characterized by the absence or paucity of the granules, 
so that broad, ill defined light lines mark off adjacent masses of proto- 
plasm from one another. These lines are so related to the indentations 
in the deep surface of the layer, and to the arrangement of the nuclei, 
as to leave no doubt that they correspond in position to the cell walls 
which were visible during the earlier stages. The nuclei have now 
become more flattened than in earlier stages, and are quite eccentric in 
position, being much nearer the deep than the outer surface of the 
hypodermis. Each nucleus exhibits a single deeply staining nucleolus 
and a number of scattered chromatin granules. 
The hypodermis has already begun to secrete the chitinous cuticula 
of the wing membrane (cta. a/.), but it is as yet very thin. It becomes 
much thicker as the wings develop. . 
The next stage (Plate 3, Fig. 12) represents the condition found in a 
pupa of Danais plexippus about a week before emergence. It is only a 
few hours older than the stage just described. It will be seen that 
a considerable change in the hypodermis cells has nevertheless taken 
place. Each sends out from its deep surface a process (fbr. h’drm.), 
