The First Abdominal Segment of Embryo Insects. 
95 
Xiphidium ensiferum. Scud. 
(Plate 2, Fig. 12, 13, 14.) 
The ontogeny of this interesting insect presents a remarkable and ap¬ 
parently isolated retention of many annelid traits. Among other peculari- 
ties there is developed in an early stage a large and rounded preoral disk 
between the procephalic lobes, making the head of the embryo resemble a 
clover leaf. This preoral disk is soon completely constricted off from the 
body of the embryo proper, and, moving forward a short distance, gives 
rise to two cellular envelopes. The movements of the embryo in relation to 
the yolk also differ markedly from anything heretofore described in insect 
development. As I shall devote a special paper to a description of 
the development of the Xiphidium embryo, I will here confine my 
attention to the pleuropodia which are quite as prominently developed as 
in other Orthoptera. 
The embryo when first formed on the convex surface of the curved 
elongate- oval egg, resembles very closely the Blatta embryo which I have 
figured in a corresponding stage [’89c, Plate XVII, Fig. 45]. The appendages 
of the first abdominal segment arise as in Blatta , but as soon as the differen¬ 
tiation of their component cells sets in, a great difference between the 
Blattid and Locustid pleuropodia becomes apparent. Each of the modified 
appendages becomes bulbous and constricted into a peduncle at its base; 
the contour, however, is not evenly rounded but somewhat angular. The 
distal end of the sack terminates in a point. Sections show that the 
cavity of the organ is very large [Fig. 12, cv.] while the cells forming the 
walls are consequently reduced to short and broad prisms. Their cytoplasm, 
though still distinctly granular, is paler than that of the ectoderm cells of 
the thoracic appendages and body walls. The nuclei, too, stain much less 
deeply than the much smaller nuclei of the remaining ectoderm, presum¬ 
ably because the quantity of cvtochylema is relatively much greater, while 
the amount of chromatin in the modified and unmodified ectoderm re¬ 
mains approximately constant. At first the large cavity of the pleuropodium 
communicates with the body cavity by means of a canal through the 
peduncle; later this communication seems to be completely cut off by the 
disappearance of the lumen. 
While the cells of the pleuropodia are differentiating to reach the 
stage figured [Fig. 12] and described, the embryo passes through the 
yolk backwards and emerges tail first on the concave surface of the egg. 
Here it grows considerably and then during revolution passes around the 
posterior pole of the egg and again makes its appearance on the convex 
surface of the yolk. During the time that the embryo is going through 
these peculiar maneuvers, the pleuropodia reach their maximum size and 
advance towards the pleurae. Henceforth they diminish in size while their 
peduncles become thinner. The oldest embryos examined had their eyes 
