AND MORPHOLOGY OF APHIS. 209 



specimens the cell-cavities of the inner portion of the thickening were particularly well 

 marked ; and tlie 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 pseudovmn to an ovum 

 is completed by the formation of a pseudovitellinc membrane (fig. 3, 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 acquii-es greater thickness and strength as development proceeds. 



The embryo first becomes clearly fashioned in pseudova between j^oth and rioth of an 

 inch in length (PI. XXXVII. fig. 6). At the distal extremity, in the region of the thickening 

 of the blastoderm, the latter appears separated into two portions, the outer of which foi-ms 

 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 laiwa, 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 whoUy formed by the pseudovitellus. The latter is 

 aggregated into a few large globular masses, wMch are in immediate contact with the 

 pseudovitelline membrane on their dorsal surface. 



The pseudovitellus is in immediate contact inferiorly with a layer of the blastoderm of 

 a more pellucid aspect than the rest, and separated from it by a more or less distinct line 

 of demarcation. This layer {q) could be detected only on the dorsal face of the thoracic 

 and cephalic segments, and owed its superior transparency to the compai-atively large 

 size of the clear cavities surrounding its endoplasts. 



That portion of the layer which covered the posterior portion of the thoracic segment 

 was particulai'ly remarkable for the size and clearness of its cells and their endoplasts {/•). 

 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 (PI. XXXVIII. 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 caeca. It would be a great mistake to suppose that 

 it is only one of these chambers, however ; it is in fact the radiment of an entire caecum ; 

 and before the embryo leaves the parent, it becomes divided into three chambers by the 

 gradual development and metamorphosis of pseudova in the way described above. 



