The First Abdominal Segment of Embryo Insects. 
93 
greater portion of the body is still filled with yolk [not seen in the 
figure], while a portion of the mesoderm has been converted into connective 
tissue, ms. The pleuropodium is undergoing degeneration. We have seen 
that in Blatta the nuclei of the huge cells so characteristic of the organ are 
at first distal in position, but that they gradually wander back towards the 
body during the breaking down of the appendage (Fig. 8). A like migra¬ 
tion of nuclei seems to occur in Periplaneta, though I cannot affirm this 
with certainty as I have seen only pleuropodia in the stage figured. Cer¬ 
tain it is, however, that many of the nuclei lie in the proximal ends of the 
cells. The shape of cells so closely applied to one another as those under 
consideration must be determined to some extent by the position of 
their large nuclei. Now in Fig. 10, the broad ends of the cells are 
directed towards the body, while the narrow ends converge at a point [i t] 
in the distal end of the appendage. This is probably not the original arrange¬ 
ment, which would be like that seen in Blatta [Fig. 3]. In order to reach 
the condition of the advanced pleuropodial cells of Periplaneta , from the 
condition seen in Blatta in a younger stage, w T e have only to suppose that 
most of the nuclei migrate to the proximal ends of their respective cells 
and push before them a quantity of protoplasm; in this way the cells would 
become rounded off at what were originally their pointed ends, and taper 
to thread like points at what were before their broad distal ends. At a? two 
such cells are seen in the act of loosening themselves from the main mass 
and are apparently about to pass into the body cavity through the broad 
lumen of the peduncle. The cells with more deeply stained nuclei at y, 
agreeing with the cells of the ectoderm, ecd, and contrasting with the 
large pleuropodial cells, seem to belong to the common ectoderm; they at 
first, perhaps, form the walls of the peduncle but subsequently pass up into 
the appendage. This supposition is rendered more probable by their per¬ 
ipheral position. The ends of the large cells are frequently filled with and 
separated from one another by vacuoles, some of which are very large and 
conspicuous. The protoplasmic reticulum in the neighborhood of these 
vacuoles has the striated appearance described above for Blatta. At the 
distal end of the pleuropodium the protoplasm is less compact and in many 
of my sections, like the one figured, spreads out in an irregular granular 
mass which leaves the appendage and probably joins the amniotic coagulum. 
If my interpretation of the few stages which I have seen is correct, the 
pleuropodia of Periplaneta disappear partly by absorption into the body of 
the embryo and partly by the dissolution of their outer portions be¬ 
tween the body walls and the egg-envelopes. In the section figured the 
cuticle [ct] is formed on the pleural and ventral walls of the abdomen, 
but is not continued over the surface of the pleuropodium. 
