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The First Abdominal Segment of Embryo Insects. 101 
lent [saftigen] cells. Later, when the back of the embryo has closed over, 
these appendages atrophy [“ gehen zu Grunde”]. They fall off and disap¬ 
pear completely. [“ Sie fallen spurlos ab”]. 
It may be doubted, judging from observations on other Orthoptera, 
whether the pleuropodia arise laterad to the other appendages. Probably 
the earliest stages in their formation escaped Korotneff. His last observa¬ 
tion, viz.: that the pleuropodia fall off, is quite definite, and, if based on 
observation, precludes their possible disappearance by absorption. 
Oecanthus niveus, Serville. 
The second insect in which pleuropodia were described was Oecanthus 
niveus. Ayers [’84] while pursuing the general ontogeny of this Gryllid 
was evidently impressed with the interest attaching to these organs; hence 
the more complete account which I reproduce in full: 
“ The respiratory function of the embryo is first indicated at the time of 
revolution by the appearance of paired lateral outgrowths of the ectoderm 
from the pleural region of the first abdominal segment. These gills or 
respiratory organs come to lie just behind, but dorsad of the base of the 
third thoracic appendage. In outline they are broadly oval or kidnev- 
shaped and are united to the body by a short peduncle springing from the 
center of that face of the disc which is in contact with the body of the 
embryo. These folds are cellular structures and at different periods are 
solid or hollow. The cells of the folds early lose their ectodermic charac¬ 
ters and become somewhat larger than those of the adjacent body wall. 
In the fresh condition they appear enucleate and coarsely granular, but 
upon treatment with osmic or acetic acid a nucleus is distinctly visible. 
In surface view there is to be seen a clear central area which indicates the 
position of the internal cavities of the gill. These cavities are continuous 
with the body cavity and probably serve as channels through which the 
vascular fluid circulates They vary in shape and relative proportions. 
The relations of these appendages to the body is best seen in sections. The 
outgrowing flap is here seen to project over an invagination immediately 
below it and in some instances to become apposed so closely to the body 
wall as to convert the open pocket into a closed canal. In its middle part, 
where the fold fuses with the body, its cells are separable into two irregular 
layers which correspond to the two primitive plates of the fold, but they 
fuse completely, or become widely separated, in the free portion of the 
pad. These appendages reach their greatest degree of development soon 
after the revolution of the embryo, and then gradually atrophy, entirely 
disappearing before the complete closure of the body walls. In sections of 
the gill organ before its atrophy [or absorption] one finds both distinct 
canals and lacunar spaces, which radiate from the point of connection of 
the pad with the body, and these together with the arrangement of the 
cells give the radiate structure characteristic of the fresh gill. The canals 
