On the nineteenth day, Mr. Newport found that the animals had acquired a little darker colour, but were still remaining quiet in their cells, and did not appear to have taken food. The enlargement of the body had not extended to the prothorax, which did not increase in size in proportion to the rest. The double pairs of new legs to the sixth and seventh segments were now distinctly visible through the exter nal tegument, which had begun to be separated from the under surface of the old segments, to which up to this period it had closely adhered. The patch on the side of the seventh segment had become darker, and the new segments were further advanced.
On the day (fig. 323) the young Juli still remained coiled up and perfectly quiescent, with their legs disposed side by side along the under surface of the body, like the pupae of dopterous Insects. The new legs had ably increased in size, as well as the whole animal, although it had not taken food. The mal was still partially coiled up, but the skin that covered its body was greatly distended, more especially along the ventral surface. It was less able to move than before, the period of throwing off this skin being fast ing : the double legs of the sixth and seventh segments, inclosed in their proper skin, were now more elongated and very much en larged, and the new segments were further developed as well as the germinal membrane. The external tegument was more extensively separated from the whole body, especially at the posterior part, and the head was retracted within it and bent on the under part of the thorax. It was thus evident that this tegu ment was not of recent formation, that it simply enclosed the animal as the whole had been previously enclosed in the amnion, as is proved by the circumstance that it extended smoothly over the whole body, antenna: and legs, and did not follow the inflection or redu plication of the proper surface of the animal like the true skin beneath it, but passed di rectly over the segments, and was simply pro truded or distended by the growth of parts be neath, as in the instance of the new legs (b). Up to this period, therefore, observes Mr. New port, the young Julus must still be regarded as in the embryo condition, although for a day or two after bursting the amnion, it possessed the power of locomotion and evinced some developement of instinct. At its next change of skin, when it enters what Mr. Newport regards as the fourth period of its developement, and when it has acquired fourteen pairs of legs, it assumes for the first time a condition analogous to the larva state of true insects on bursting from the ovum ; the difference be tween the two being that the analogue of this tegument of the embryo in insects is slipped off at the bursting of the amnion on leaving the shell, while that of the Myriapod is not thrown off until some days after it has entirely left the ovum. This embryo condition of the animal will therefore explain the circumstance of its first acquiring a slight power of locomotion, and then remaining perfectly quiescent without taking food to prepare for this change—the third period of its embryo life.
The lower portion of the alimentary canal is at this time distinctly visible through the new segments, exhibiting a corrugated or folded ap pearance, an arrangement doubtless intended to allow of its sudden extension at the period of throwing off the skin and elongation of the seg ments. The colon is of a very dark colour and
exhibits its thickened peculiar structure with its longitudinal muscular bands. Around its posterior part, Mr. Newport observed an aggre gation of what appeared to be globular cells. They seemed to be part of the organs of gene ration in the course of developement. At first they were regarded as hepatic vessels, but this Mr. Newport considers could hardly be the case from the fact that each of these organs directly enters the canal as a straight vessel, but they might be vessels folded up to be unfolded suddenly, as in the case of the alimentary canal.
By the twenty-sixth day the young Julus casts off the covering in which it had hitherto been infolded, and enters the fourth period of developement, having now sevenpairs of legs and fifteen segments to its body (fig. 324).
In this condition the antenna; were found to have become elongated by at least one-third of their original length, and exhibited six distinct joints. The eye still consisted of a single ocel lus, but this was now surrounded by a darker coloured portion of the tegument. The new legs (b c) were equal in size and length to the r original ones, but were evidently more feeble. The transverse markings on the seven anterior segments (2-7) were very distinct, and the large brown patch on the seventh segment was much darker in colour. The whole body of the ani mal was considerably elongated. This was produced chiefly by the extension of the new segments (7 g) formed by the germinal mem brane at the posterior part of the seventh, and which, in the early part of the last period, seemed to form a single distinct segment co vered by the common tegument. The most anterior of these segments (8), now the eighth of the whole body, had acquired an extent equal on its upper surface to the preceding seg ment, but was shorter on its ventral surface. Like the preceding original segments it was divided into two regions by a transverse de pressed line. The next segment in succession to this, the ninth, had also become enlarged to about one-third of the size of the eighth, and was, like it, marked transversely. The next four segments were each more developed than in the preceding state, but not to so great an extent as the others. The two remaining seg ments (14 15), the penultimate and anal, had undergone no change. They had simply ac quired a little extension at the apex of the seg ment, and were now covered with a few scat tered hairs. It is thus proved that the body is elongated, not by the division of the newly formed segments into others, but always by the formation of new ones in the germinal mem brane that extends from the posterior margin of the antepenultimate segment to the penultimate, which last segment, with the anal, undergoes no change. That segment is always furthest ad vanced in growth which is immediately poste rior to the last segment that possesses legs, and then the next in succession, until we arrive at the terminal ones—the penultimate and the anal—that never possess legs.