_Dorsal Closure._--The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. The method of this dorsal closure varies in different insects. In the Colorado beetle (_Doryphora_), whose development has been studied by W. M. Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. V. Graber has described a similar method of dorsal closure in the saw-fly _Hylotoma_.
[Illustration: After Heymons, _Zeit. Wiss. Zoolog._ vol. 53.
FIG. 19.--Cross sections through Abdomen of German Cockroach Embryo. A (later than fig. 16) magnified. B (still more advanced, dorsal closure complete) magnified.
ec, Ectoderm. en, Endoderm. sp, Splanchnic layer of mesoderm. y, Yolk. h, Heart. p, Pericardial septum. c, Coelom. g, Germ-cells surrounded by rudiment-cells of ovarian tubes. m, Muscle-rudiment. n, Nerve-chain. f, Fat body. s, Inpushing of ectoderm to form air-tubes. x, Secondary body-cavity.]
_Mesoderm, Coelom and Blood-System._--From the mesoderm most of the organs of the body--muscular, circulatory, reproductive--take their origin. The mass of cells undergoes segmentation corresponding with the outer segmentation of the embryo, and a pair of cavities--the coelomic pouches (fig. 16, M)--are formed in each segment. Each coelomic pouch--as traced by Heymons in his study on the development of the cockroach (_Phyllodromia_)--divides into three parts, of which the most dorsal contains the primitive germ-cells, the median disappears, and the ventral loses its boundaries as it becomes filled up with the growing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlargement and coalescence of the blood channels and by the splitting of the fat body. It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.