The First Abdominal Segment of Embryo Insects. 
9 ? 
opaque white, may be readily recognized before sectioning. It is difficult 
to remove the chorion without seriously damaging the egg, so that sections 
are best stained on the slide. The development is very much like that of 
Aphis as described by Will. [’&§.] This need not surprise us when we 
stop to consider the close relationship of the Phytophthora and Homoptera. 
In the earliest stage examined the embryo was found in the middle of 
the yolk, with the thoracic appendages just making their appearance, and 
the thin amnion so closely applied to the ventral plate as to be difficult of 
detection in some sections. The embryo is quite straight, exhibiting little 
of the pronounced curvature of the Aphis embryo. 
In a later sta;e when the thoracic and cephalic appendages are well 
established, no appendages are to be observed on the abdominal segments, 
sections through which show that the ventral surface is very flat, without 
even the bulgings that have frequently been mistaken for rudimental ap¬ 
pendages. In a cross section through the middle of the first abdominal 
segment [Fig. 20] there is seen at the points corresponding with the places 
of evagination of the metathoracic appendages of the preceding segment, 
a pair of ectodermic thickenings [ ap ]. The cells of these thickenings as 
shown by the curvature of their nuclei are aggregated somewhat like the 
segments of an orange. The long axes of all the ectoderm cells are in this 
stage directed dorso-ventrally. I have for the sake of emphasis repre¬ 
sented the thickenings, which for reasons given below, I believe to be true 
homologues of the evaginated pleuropodia of other insect embryos, as 
paler than the cells of the surrounding ectoderm, though in reality no such 
differentiation has as yet set in. In the section figured a slight depression 
marks the convergence of the outer ends of the pleuropodial cells. 
During the revolution of the embryo the pleuropodium reaches its full 
size and presents in section the appearance of Fig. 19 ap. It is easy to see 
how this organ originates from an orange shaped cluster of cells like that 
just described. The ectodermic elements increase greatly in length and 
assume the form of curved pyramids with their tapering apices attaining 
the surface of the body and their broadened nuclear ends projecting into 
the body cavity. The outer and attenuated ends of the cells are uniformly 
hyaline and stain very faintly in borax carmine. The cytoplasm of the 
inner ends is granular like that of the remaining ectoderm, [ecd.] The 
nuclei of the pleuropodium seem not to differ in their finer structure from 
the nuclei of the general ectoderm. They are frequently triangular or vi¬ 
olin-shaped both in the pleuropodium and in the undifferentiated ectoderm. 
The only difference is one of position: the nuclei of the body wall lie at right 
angles to their former position. A granular mass, the amniotic secretion, 
fills the space between the body walls and the egg membranes [ ch .] In one 
place, however, this mass is replaced by one of a different nature, a glairy, 
homogeneous and vacuolated substance [s] of irregular though rounded 
outline, firmly attached to the attenuated tips of the pleuropodial cells. 
This homogeneous mass, which stains pink in borax carmine, is often more 
7—A. & L. 
