ON THE AGAMIC REPRODUCTION AND MORPIIOLOGY OF APHIS 47 
~ cells ;” but the thickening shows itself, especially upon one side of 
the distal end of the germ (that turned towards the vagina), where 
the blastoderm is nearly twice as thick as in other parts. A linear 
demarcation appears in the midst of this thickened layer (fig. 4); and 
at the same time indications of a separation are traceable between 
the distal extremity of the thickened portion and the rest of the 
blastoderm: it is as if the latter were giving way at this point. In 
some specimens the cell-cavities of the inner portion of the thicken- 
ing were particularly well marked ; and the coarsely granular central 
substance exhibited a tendency to break up into large globular masses, 
which became particularly distinct 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 pseudo- 
vitelline membrane (fig. 3, a). This structureless homogeneous mem- 
brane is, doubtless, developed by a process of excretion, either from 
the pseudovum or from the walls of the chamber which contains it. 
It completely envelopes the pseudovum, and acquires greater thickness 
and strength as development proceeds. 
The embryo first becomes clearly fashioned in pseudova between 
sigth and ~4,5th of an inch in length (Pl. NNXVII. [Plate 2] fig. 5). 
At the distal extremity, in the region of the thickening of the blasto- 
derm, 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 
blastoderm. 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 segment, 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. 
The pseudovitellus is in immediate contact inferiorly with a layer 
