AND MORPHOLOGY OF APHIS. 209 
specimens the cell-cavities of the inner portion of the thickening were particularly well 
marked ; and the coarsely granular central substance exhibited a tendeney to break up into 
large globular masses, which became particularly distinet on the addition of water. 
It is in the largest of these germs that the resemblance of the pseudovum to an ovum 
is completed by the formation of a pseudovitelline membrane (fig. 8, a). This structureless 
homogeneous membrane is, doubtless, developed by a process of excretion, either from 
the pseudovum or from the walls of the chamber which contains it. It completely enve- 
lopes the pseudovum, and acquires greater thickness and strength as development proceeds. 
The embryo first becomes clearly fashioned in pseudova between „45th and 41th of an 
inch in length (Pl. XX XVII fig. 5). At the distal extremity, in the region of the thickening 
of the blastoderm, the latter appears separated into two portions, the outer of which forms 
a sort of hood over the inner. The hood eventually becomes the hinder part, if not the 
whole, of the abdomen of the larva. It is continuous, on the side answering to the dorsal 
side of the larva, with the rest of the blastoderm, which now, instead of enclosing the 
pseudovitellus, lies partly beneath and partly behind it. That portion of the blastoderm 
which lies behind the pseudovitellus, and parallel with the hood, is the rudiment of the 
sternal region of the thorax ; and I shall hereafter term it the thoracic segment of the blas- 
toderm. That part of the blastoderm which lies beneath the pseudovitellus will become 
the sternal region of the head; and I shall therefore call it the cephalic segment, while 
the hood itself is the abdominal segment of the blastoderm. 
The thoracic segment, it will be observed, is in this stage bent up at right angles to 
the axis, and reaches the dorsal region, which it bounds posteriorly. The cephalic seg- 
ment, on the other hand, hardly extends upwards at all, but lies in one plane; so that the 
anterior end of the embryo is almost wholly formed by the pseudovitellus. The latter is 
aggregated into a few large globular masses, which are in immediate contact with the 
pseudovitelline membrane on their dorsal surface. - Å 
The pseudovitellus is in immediate contact inferiorly with a layer of the blastoderm of 
å more pellucid aspect than the rest, and separated from it by a more or less distinct line 
of demarcation. This layer (g) could be detected only on the dorsal face of the thoracic 
and cephalic segments, and owed its superior transparency to the comparatively large 
` size of the clear cavities surrounding its endoplasts. 
That portion of the layer which covered the posterior portion of the thoracic segment 
Was particularly remarkable for the size and clearness of its cells and their endoplasts (7). 
In the progress of development, the central portion of the alimentary canal occupies a 
place nearly corresponding to the centre of the clear layer; while, if we trace out the site of 
the rest of the mass in larger and larger embryos (Pl. XXX VII. figs. 1, 3, 4, 5), we find 
it always retaining the same relative position to the reflected abdominal hood, but gradu- 
ally enlarging, and eventually becoming subdivided into five oval lobes upon each side, 
each of which surrounds itself with a membrane, and assumes the form of the terminal 
Chamber of one of the pseudovarial emca. It would be a great mistake to suppose that 
It is only one of these chambers, however ; it is in fact the rudiment of an entire cæcum ; 
and before the embryo leaves the parent, it becomes divided into three chambers by the 
Sradual development and metamorphosis of pseudova in the way described above. 
