This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* Memoires pour servir a l'Histoire des Insectes. 7 vols. 4to. Stockholm, 1778. 1 Osservazioni per servire alia storia di una specie di Julus comunissima. Bologna, 1817.
(732). The development of the young Julus has been traced more recently by Mr. Newport with great care; and the result of that gentleman's observations relative to this part of the history of the Myriapods is of extreme interest, both to the physiologist and in an entomological point of view.
(733). The embryo, when it first becomes distinguishable in the interior, of the ovum, is entirely destitute of limbs, or of any appearance of segmental division, and even at the moment of its escape from the egg, which is effected by the laceration of the egg-shell, but very faint traces of segmentation are discernible. After its extrusion, however, its growth advances with considerable rapidity, and it soon becomes visibly divided into eight distinct segments, including the head (fig. 141, a) - the ninth or anal segment (d) being still indistinct. The four thoracic segments, moreover, now exhibit on their ventral surface little nipple-shaped protuberances, three of which on each side are the rudiments of future legs. No internal viscera are as yet distinguishable, the whole embryo being still a congeries of vesicles, or cells, in the midst of which some faint traces of a future alimentary canal seem to be indicated. In this state the body of the embryo is completely enclosed in a smooth and perfectly transparent membrane (fig. 141, a, e), which seems to contain a clear fluid. This membrane Mr. Newport regards as the analogue of the amnion - the vitelline or investing membrane of the embryo in the higher animals, and identical with the membrana vitelli, or proper membrane of the yelk. It is a shut sac that completely invests the embryo, except at its funnel-shaped termination at the extremity of the body (fig. 141, a, d), where it is constricted, and, together with another membrane (which in the unburst egg is external to this and lines the interior of the shell), assists to form the cord or proper funis (d) that enters the body of the embryo at the posterior part of the dorsal surface of the future antepenultimate segment, where the mucro or spine exists in the adult animal. (734.) A new process is now about to commence - the development of new segments. Up to the present period the posterior part of the body remains less distinctly divided into segments than the anterior, the first five segments being the most distinctly marked; the sixth and seventh now become more defined. It is in the membrane (fig. 141, c,f) that connects the seventh with the eighth segment (at the posterior margin of which last the funis (d) enters, and which is permanent as the penultimate segment throughout the life of the animal) that the formation of new segments is taking place. At this period it is only a little, illdefined space, that unites the seventh and eighth segments into one mass; but in proportion as the anterior parts of the body become developed, this part is also enlarged, not as a single structure, but as a multiplication or repetition of similar structures.
Fig. 140. Growth of young Julus terrestris. (After De Geer).
Fig. 141. Development of the embryo in Julua terrestris. (After Newport).
(735). About the seventeenth day the little embryo is ready to leave the amnion in which it has been hitherto enveloped. Its body is found to have become considerably elongated, the increase of length being mainly occasioned by the growth of the posterior segments, but more especially by the development of new ones, which now begin to make their appearance in the antepenultimate space (fig. 141, c, f), which is, in fact, the proper germinal space or germinal membrane, whereby the production of all the future segments is effected. The seven anterior segments, including the head, are now greatly enlarged, and the hitherto minute penultimate and anal segments (8,9) become much enlarged, and rapidly acquire the form they afterwards retain through the life of the animal. This latter fact shows that it is not merely by an elongation and division of the terminal segment that the body of the Julus is developed, but that it arrives at its perfect state by an actual production of entirely new segments, the formation of which is in progress long before they are apparent to the eye, and that the original segments of the ovum into which the animal is first moulded are permanent.
(736). The manner in which new legs are produced is equally curious. Up to the present period the animal is furnished with only three pairs (fig. 141, c, b c), but four additional pairs are nevertheless in progress of formation. These, at present, exist only as eight minute nipple-shaped prominences on the under surfaee of the sixth and seventh segments (fig. 141, c, 6, 7), four on each, covered by the common integument, which, as in the larval condition of insects, is a deciduous membrane. The newly-formed legs, however, go on rapidly increasing in size until about the twenty-sixth day, when, throwing off the skin in which it has hitherto been encased, the young Julus presents itself with seven pairs of legs and a body consisting of fifteen segments (fig. 141, E).
(737). In this condition the body of the animal still continues to elongate, not by the division of the already-formed segments into others, but always by the formation of new ones in the germinal membrane that extends from the posterior margin of the antepenultimate segment to the penultimate, which last segment, as well as the anal, undergoes no change; and it may likewise be observed that that segment of the newly-formed portion of the body is always furthest advanced in growth which is immediately posterior to the last segment which possesses legs, - and then, the next in succession, until we arrive at the terminal ones (the penultimate and the anal), that never have legs appended to them.