Fig. 81.—Early stages of the segmentation of a beetle (Lina): A, segmentation not visible, 1 day; B, segmentation of head visible; C, segmentation still more advanced, 2¼ days; PC, procephalic lobes; g1, g2, g3, segments bearing appendages of the head; th, thorax; th1, th2, th3, segments of the thorax; a1, a2, anterior abdominal.
Turning our attention to the origin of the segmentation, that is so marked a feature of Insect structure, we find that evidence of division or arrangement of the body into segments appears very early, as shown in our Figure of some of the early stages of development of Lina (a beetle), Fig. 81. In A the segmentation of the ectoderm has not commenced, but the procephalic lobes (P C) are seen; in B the three head segments are distinct, while in C the thoracic segmentation has occurred, and that of the abdomen has commenced. Graber considers that in this species the abdomen consists of ten segmental lobes, and a terminal piece or telson. According to Graber[[77]] this is not a primitive condition, but is preceded by a division into three or four parts, corresponding with the divisions that will afterwards be head, thorax, and abdomen. This primary segmentation, he says, takes place in the Hypoblast (Endoderm) layer of the ventral plate; this layer being, in an early stage of the development of a common grasshopper (Stenobothrus variabilis), divided into four sections, two of which go to form the head, while the others become thorax and abdomen respectively. In Lina the primary segmentation is, Graber says, into three instead of four parts. Graber's opinion on the primary segmentation does not appear to be generally accepted, and Wheeler, who has studied[[78]] the 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 determining by means of a section the real number of segments.
Fig. 82.—Embryo of a moth (Zygaena) at the fifth day (after Graber): am, amnion; s, serosa; p, procephalic lobes; st, stomodaeum; pr, proctodaeum; g1, g2, g3, the mouth parts or head appendages; th1, th2, th3, appendages of the thoracic segments; a1-a10, abdominal segments; s.g, salivary gland.
The later stages in the development of Insects are already proved 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 (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 yolk-cells associate themselves with the mesoblast cells, so that the constituents of the mesoblast have a twofold origin." Wheeler finds[[79]] that amoeboid cells—he styles them vitellophags—traverse the yolk and assist in its rearrangement; he insists on the importance both as regards quantity and quality of the yolk.
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-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 circumference. But 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 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.
