1915] Middle Membrane in Wings of Platyphylax 207 
with certainty be found. One does however find, running 
through the middle of each section, a continuous, narrow, lighter 
_ zone which is connected with and separates the two layers of 
hypodermis from each other (Fig. 3). This zone is without 
definite boundaries, and, from the darker protoplasm of the 
adjacent hypodermal layers, there are numerous small processes 
extending into it so that it is impossible to trace any definite line 
which might separate the parts from each other. Another 
change one notices at this stage is in the gradual moving of the 
nuclei of each layer towards the outer surfaces of the wing; most 
of these nuclei are no longer, as formerly, grouped in the basal 
half of each layer but are now fairly well scattered in all its 
parts. This is well shown by comparing figures one and three. 
It is apparent that this hght median zone remains for but a 
short time during the development of the wing. Slides through 
wings a little older than the last described fail to show any 
median zone which can be recognized as lighter in shade than 
the rest of the wing, but, in the same median position, one can 
still see the same zone but it is now darker than the rest; the 
sections showing this were prepared and stained in the same 
way as the slides showing the lighter median zone. The two 
layers of the hypodermis no longer lie close against this median 
zone but have moved slightly away from it, not leaving a clear 
entirely open space, but each layer of the hypodermis is con- 
nected with the median zone by numerous protoplasmic strands 
separated from each other by vacuoles which are irregular in 
outline. This makes each side of the median zone much lighter 
and this contract coupled with a probable increase in the density 
of the protoplasm of the middle zone, might account for its now 
appearing darker than the other parts of the slide (Fig. 4). | 
There are now, excepting the cuticular covering, five layers 
shown in each section of the wing: 1, two outer layers, the 
original hypodermis, which have decreased in width and now 
form but a part of the entire section; 2, along the inner surface 
of each of these is a lighter layer composed of numerous vacuoles 
separated from each other by protoplasmic strands which con- 
nect the two outer layers, 1, with 3, a median layer which 
appears slightly darker than the other parts of the section. 
None of these layers has a distinct boundary. This last median 
layer is present in the developing wing of Platyphylax and 
forms what is commonly known as the middle membrane; this, 
