the bristle-tail _Lepisma_ and in dragon-flies they give rise to the mid-gut. These views are not, however, supported by other recent observers. J. Carriere's researches (1897) on the embryology of the mason bee (_Chalicodoma_) agree entirely with the interpretations of Kowalevsky and Heider, and so on the whole do those of F. Schwangart, who has studied (1904) the embryonic development of Lepidoptera. He finds that the endoderm arises from an anterior and a posterior rudiment derived from the "endoblast," that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901), after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. On the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some Hexapoda the whole digestive tract may be ectodermal. It must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the "germ-layer theory." Heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a "latent endoderm-group" in those invaginations. [Illustration: From Nussbaum in Miall and Denny, _The Cockroach_, Lovell Reeve & Co. FIG. 16.--Cross section of Embryo of German Cockroach (_Phyllodromia_). S, serosa; A, amnion; E, ectoderm; N, rudiment of nerve-cord; M, mesodermal pouches.]