[Illustration: 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^1_, _g^2_, _g^3_, the mouth parts or head appendages; _th^1_, _th^2_, _th^3_, appendages of the thoracic segments; _a^1_-_a^{10}_, 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 {152}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.