embryology of another Orthopteron, considers it will prove to be incorrect. When the secondary segmentation occurs the anterior of the two cephalic divisions remains intact, while the second divides into the three parts that afterwards bear the mouth parts as appendages. The thoracic mass subsequently segments into three parts, and still later the hind part of the ventral plate undergoes a similar differentiation so as to form the abdominal segments; what the exact number of these may be is, however, by no means easy to decide, the division being but vague, especially posteriorly, and not occurring all at once, but progressing from before backwards. The investigations that have been made in reference to the segmentation of the ventral plate do not at present justify us in asserting that all Insects are formed from the same number of embryonic segments. The matter is summarised by Lowne, to the effect that posterior to the procephalic lobes there are three head segments and three thoracic segments, and a number of abdominal segments, " rarely less than nine or more than eleven." It will be seen by referring to Figure 81 that the segmentation appears, not simultaneously, but progressively from the head backwards; this of course greatly increases the difficulty of determin- p. ing by means of a section the real number of segments. st да The later stages in the develop- gu ment of Insects are already proved ge to be so various that it would be impossible to attempt to follow them in detail; but in Fig. 82 'we represent a median section of the embryo of Zygaena filipendula at the fifth day. It shows well some of the more important of the general features of the development at a stage subsequent to those represented in Fig. 81, A, B, C. The very distinct stomodaeum the th3 pr am FIG. 82.-Embryo of a moth (Zygaena) at the fifth day (after Graber): am, amnion; s, serosa; p, procephalic lobes; st, stomodaeum; pr, proctodaeum; g', 92, 93, the mouth parts or head appendages; th1, th2, th3, appendages of the thoracic segments; al-alo, abdominal segments; s.g, sali. vary gland. (st) and proctodaeum (pr) are seen as inflexions of the external wall of the body; the segmentation and the development of the ventral parts of the embryo are well advanced, while the dorsal part of the embryo is still quite incomplete. The method of investigation by which embryologists chiefly carry on their researches is that of dividing the egg after proper preparation, into a large number of thin sections, which are afterwards examined in detail, so as to allow the arrangement to be completely inferred and described. Valuable as this method is, it is nevertheless clear that it should, if possible, be supplemented by direct observation of the processes as they take place in the living egg: this method was formerly used, and by its aid we may still hope to obtain exact knowledge as to the arrangements and rearrangements of particles by which the structures develop. Such questions as whether the whole formative power in the egg is absolutely confined to one or two small centres to which the whole of the other egg contents are merely, as it were, passive accessories, or whether an egg is a combination in which some portion of the powers of rearrangement is possessed by other particles, as well as the chromosomes, in virtue of their own nature or of their position at an early period in the whole, can scarcely be settled without the aid of direct observation of the processes during life. The importance of the yolk is recognised by most of the recent writers. Nussbaum states (loc. cit.) that "scattered yolkcells associate themselves with the mesoblast cells, so that the constituents of the mesoblast have a twofold origin." Wheeler finds that amoeboid cells he styles them vitellophagstraverse the yolk and assist in its rearrangement; he insists on the importance both as regards quantity and quality of the yolk. 1 The eggs of some insects are fairly transparent, and the process of development in them can, to a certain extent, be observed by simple inspection with the microscope; a method that was used by Weismann in his observations on the embryology of Chironomus. There is a moth (Limacodes testudo), that has no objection to depositing its eggs on glass microscope-slides. These eggs are about a millimetre long, somewhat more than half that width, are very flat, and the egg-shell or chorion is very thin and perfectly transparent. When first laid the contents of this egg appear nearly homogeneous and evenly distributed, a finely granular appearance being presented throughout; but in twenty 1 J. Morphol. viii. 1893, pp. 64, 65, and 81. four hours a great change is found to have taken place. The whole superficial contents of the egg are at that time arranged in groups, having the appearance of separate rounded or oval masses, pressed together so as to destroy much of their globular symmetry. The egg contents are also divided into very distinct forms, a granular matter, and a large number of transparent globules, these latter being the fatty portion of the yolk; these are present everywhere, though in the centre there is a space where they are very scanty, and they also do not extend quite to the circumBut the most remarkable change that has taken place is the appearance in the middle of the field of an area different from the rest in several particulars; it ference. occupies about one-third of the width and one-third of the length; it has a whiter and more opaque appearance, and the fat globules in it are fewer in number and more indistinct. This area is afterwards seen to be occupied by the developing embryo, the outlines of which become gradually more distinct. Fig. 83 gives an idea of the appearance of the egg about the middle period of the development. In warm weather the larva emerges from this egg ten or eleven days after it has been deposited. A B testudo about the middle of the development of the embryo; B, micropyles and surrounding sculpture of chorion. The period occupied by the develop- FIG. 83.-A, Egg of Limacodes ment of the embryo is very different in the various kinds of Insects; the blowfly embryo is fully developed in less than twenty-four hours, while in some of the Orthoptera the embryonic stage may be prolonged through several months. According to Woodworth the blastoderm in Vanessa antiopa is complete in twenty-four hours after the deposition of the egg, and the involution of the ventral plate is accomplished within three days of deposition. Metamorphosis. The ontogeny, or life history of the individual, of Insects is peculiar, inasmuch as a very large part of the development takes place only late in life and after growth has been completed. Insects leave the egg in a certain form, and in that condition they continue with, however, a greater or less amount of change according to kind-till growth is completed, when, in many cases, a very great change of form takes place. Post-embryonic development, or change of form of this kind, is called metamorphosis. It is not a phenomenon peculiar to Insects, but exists to a greater or less extent in other groups of the Metazoa; while simpler postembryonic development occurs in nearly all, as in scarcely any complex animals are all the organs completely formed at the time the individual becomes possessed of a separate existence. In many animals other than Insects the post-embryonic development assumes most remarkable and complex forms, though there are perhaps none in which the phenomenon is very similar to the metamorphosis of Insects. The essential features of metamorphosis, as exhibited in the great class we are writing of, appear to be the separation in time of growth and development, and the limitation of the reproductive processes to a short period at the end of the individual life. The peculiar phenomena of the postembryonic development of the white ants show that there exists some remarkable correlation between the condition of the reproductive organs and the development of the other parts of the organisation. If we take it that the post-embryonic physiological processes of any individual Insect are of three kinds, -growth, development, and reproduction,-then we may say that in the higher Insects these three processes are almost completely separated, and go on consecutively, the order being,— first, growth; second, development; third, reproduction. While, if we complete the view by including the processes comprised in the formation of the egg and the development therein, the series will be (1) oogenesis, or egg-growth; (2) development (embryonic); (3) growth (post-embryonic); (4) development (postembryonic); (5) reproduction. The metamorphosis of Insects is one of the most interesting parts of entomology. It is, however, as yet very little known from a scientific point of view, although the simpler of its external characters have for many ages past attracted the attention and elicited the admiration of lovers of nature. may seem incorrect to say that little is yet known scientifically of a phenomenon concerning which references almost innumer It able are to be found in literature: nevertheless the observations that have been made as to metamorphosis, and the analysis that has been commenced of the facts are at present little more than sufficient to show us how vast and complex is the subject, and how great are the difficulties it presents. There are three great fields of inquiry in regard to metamorphosis, viz. (1) the external form at the different stages ; (2) the internal organs and their changes; (3) the physiological processes. Of these only the first has yet received any extensive attention, though it is the third that precedes or underlies the other two, and is the most important. We will say a few words about each of these departments of the inquiry. Taking first the external form-the instar. But before turning to this we must point out that in limiting the inquiry to the post-embryonic development, we are making one of those limitations that give rise to much misconception, though they are necessary for the acquisition of knowledge as to any complex set of phenomena. If we assume five well-marked stages as constituting the life of an Insect with extreme metamorphosis, viz. (1) the formation and growth of the egg; (2) the changes in the egg culminating in its hatching after fertilisation; (3) the period of growth; (4) the pupal changes; (5) the life of the perfect Insect; and if we limit our inquiry about development to the latter three, we are then shutting out of view a great preliminary question, viz. whether some Insects leave the egg in a different stage of development to others, and we are consequently exposing ourselves to the risk of forgetting that some of the distinctions we observe in the subsequent metamorphosis may be consequential on differences in the embryonic development. Instar and Stadium. Figs. 84 and 85 represent corresponding stages in the life of two different Insects, Fig. 84 showing a locust (Acridium), and Fig. 85 a white butterfly. In each A represents the newly-hatched individual; B, the insect just before its perfect state; C, the perfect or imago stage. On comparing the two sets of figures we see that the C stages correspond pretty well as regards the most important features (the position of the wings being unimportant), that the A stages are moderately different, |