The First Abdominal Segment of Embryo Inserts. 
91 
perficial and does not divide the cells transversely. The nuclei are seen to 
have moved back from the periphery of the appendage and to lie, several 
rows deep in the basal half of the apical, and in the apical half of the 
basal metameres into which the organ has been constricted. The contour 
of the inner ends of the prismatic cells has become more distinct and the 
cavity into which the lumen of the peduncle opens has become larger. 
The peripheral ends of the prismatic cells are full of oval vacuoles of about 
equal size, placed side by side. They occupy the whole surface of the apic¬ 
al metamere. [Fig. 6 v.~\ Perhaps these vacuoles, as also tliose described 
in a preceding stage, may be caused by the action of reagents, though the 
regularity of their occurrence and the lack of vacuoles in other tissues of 
embryo Blatter killed and prepared according to a method described in a 
former paper [’89c], seems to preclude the belief that they are artefacts. Be 
this as it may, their presence wrnuld seem to indicate that the cytoplasm of 
the outer ends of the prismatic cells is of a different, perhaps more sensi¬ 
tive, structure than that in the remaining portions of the cells. 
The appendage has now reached the highest point of its development and 
henceforth slowly advances towards dissolution. By the twentieth day 
[Fig. 7] it has become more or less irregular in outline. The peduncle \pd\ 
has increased in length and tenuity; the cavity [ci?.] has become irregular 
owing to the basal edges of the prismatic cells becoming ragged, and the 
constriction is disappearing. In the appendage figured it is still seen as a 
deep indentation on one side [cn.]; on the other side no traces of it are visi¬ 
ble. The shape assumed by the organ is now no longer constant in any 
two individuals. The vacuoles have either entirely disappeared as in Fig. 7, 
or they are much elongated and usually found between the prismatic cells. 
Where the lateral w T alls of the cells have disappeared their former position 
is often marked by these elongate vacuoles. From being intracellular as 
in Figs. 3 and 6, the vacuoles thus become intercellular just before disap¬ 
pearing. 
On about the twenty-fourth or twenty-fifth day the cuticle covers the 
whole surface of the embryo. No cuticle, however, forms on the surface of 
the pleuropodium, the elongate peduncle of which is constricted off by the 
developing chitinous secretion. When embryos of this age are kept for 
hours or even days in hsematoxylin or borax carmine and then washed, it is 
found that in perfect specimens none of the staining fluid has penetrated 
the cuticule; the embryo is still yellowish white, with a brilliant violet or 
red spot, visible to the naked eye just behind the base of the metathoracic 
leg on either side of the body. This is the pleuropodium, the only portion 
of the embryo which has been stained by such protracted immersion. 
Examination with higher powers shows that the pleuropodium has now 
become very irregular in outline. [Fig. 8.] The old boundaries between the 
prismatic cells have disappeared. The apical part of the appendage, still 
somewhat bulbous in shape, is a syncytium through which the nuclei ap¬ 
pear to be migrating toward the opening of what was formerly the pe- 
