Up to this period the embryo gives not the slightest evidence of spontaneous or voluntary motion. Internally it is still composed of cells of different sizes that are now in the course of producing muscular and other structures. In some parts of its body no arrangement of them seems as yet to have taken place, the cells being merely aggregated together. Cells of three very distinct sizes now exist. The dia meter of the largest of these is nearly three times that of the second size, and the second again are nearly twice and a half the size of the smallest. The smallest sized cells fill up the interspaces between the others, and appear as if breaking down to form interstitial or cellular substance, while the second sized cells are arranged in rows to form particular structures.
In the midst of these cells the alimentary canal is now nearly complete, but Mr. Newport was unable to observe its connexion with the funis. At its anal extremity it is a little dilated, and extends forward as a short straight intestine, the rectum, until it arrives at a part where a valve seems about to be formed. The diameter of the canal is there enlarged, and on its surface are three distinct longitudinal muscular hands. The so-called hepatic vessels also exist as dis tinct tubes inserted one on each side into the alimentary canal at the constricted or valve-like part above noticed. The canal is then conti nued forwards until it is again dilated into the proper stomach, and terminates or rather com mences in a narrow oesophagus. It is much longer than the body of the embryo, being con voluted or folded upon itself in its lower por tion, to adapt it to the changes that the body undergoes in the enlargement and elongation of its segments. It is not yet separated from the now forming structures by any distinct investment, either adipose or peritoneal, except only what belongs to itself; but is closely sur rounded by cells of the second and third size On the tenth day the great circulatory or dorsal vessel was distinctly seen through the amnion and skin. This doubtless had existed much earlier, although not observed. It was exceedingly well marked, but Mr. Newport was as yet unable to detect any motion in it. The head of the embryo had now begun to assume the peculiar corneous appearance common to the larva: of true insects ; its body had much increased in size, and the amnion was still co vered with microscopic drops of fluid.
On the eleventh day the head was more dis tinct, and the antennae appeared at its sides like short crescent-shaped clubs, with their terminations directed forwards. Above them the single ocelli were distinctly seen. All the segments, posterior to the third, exhibited the transverse line that indicated the division into double segments, and the posterior seg ments were much increased in size.
On the morning of the seventeenth day (fig. 321) Mr. Newport found all the embryos ready to leave the amnion.
Some of them were al ready detached from the shell ; others were still connected to it. Their increase of bulk within the last few hours had been very great. The body was now more straightened, the head less inflected under the thorax, and the eye was a dark-coloured spot above and behind the antenna'. The segments of the body were di vided by distinct reduplicatures of the proper tegument, and the legs folded side by side against the ventral surface were much further extended beneath the amnion (b, a). The trans verse divisions of the first six segments strongly marked the original segments, and the amnion, now about to burst, was tightly extended over the dorsal surface, and by the elongation of the body was rendered more distinct on the ventral. The great increase in the length of the animal was mainly occasioned by the growth of the posterior segments, more especially those in the antepenultimate space, the proper germinal space or membrane (f), the faint divisions of which into new segments were now distinctly seen through the amnion. The seven anterior segments, including the head, were greatly en larged, and the hitherto minute anal and pe nultimate segments (8, 9), in the first of which the remains of the funis (d) forms a rudimen tary spine, had also become enlarged, and were now fast acquiring the form they afterwards retain throughout the life of the animal. Some of the specimens soon threw off their covering and entered the third period of development.
The animal was now greatly enlarged, and possessed three pairs of legs, but it still lay with these newly developed legs coiled up without voluntary motion. The amnion had been fissured at its anterior dorsal surface, and slipped off backwards from the posterior seg ments, and lay at the anal extremity, while the animal itself, with its limbs coiled up, appeared as if exhausted with these its first spontaneous Obits. No other signs of animal existence were given than occasional slight movements of the antennae. The embryos thus passed from their apparently inanimate to an animated state of existence, from a condition in which they appeared merely to vegetate, endowed with no voluntary or instinctive powers, but like the vegetable formed entirely of an aggre gation of cells, totally incapable of spontaneous motion, to one in which they became active beings, gradually acquiring voluntary and in stinctive faculties both as regards the means of procuring nourishment and of preserving themselves from injury.