[177] Q. J. Micr. Sci., Vol. XXIV., page 596 (1884). [Illustration: Fig. 105.--Ventral Plate of _B. germanica_, side view. × 20.] [Illustration: Fig. 106.--Diagram to illustrate the formations of the Embryonic Membranes. _A_, amnion; _S_, serous envelope; _B_, blastoderm.] This curvature of the embryo is closely connected with the formation of the embryonic membranes. On either side of the ventral plate a fold of the blastoderm arises, and these folds grow towards each other beneath the chorion. Ultimately they meet along the middle line of the ventral plate (fig. 106), and thus form a double investment, the outer layer being the _serous envelope_, the inner the _amnion_. Between the two the yolk passes in, as has been explained above (fig. 107). [Illustration: Fig. 107.--Transverse section through young Embryo of _B. germanica_. _E_, epiblast; _M_, mesoblast; _Y_, yolk-cells.] At the same time that the embryonic membranes are forming, the embryonic layers make their appearance. The ventral plate, which was originally one-layered, forms the _epiblast_ or outer layer of the embryo, and from this are subsequently derived the middle layer (_mesoblast_) and the deep layer (_hypoblast_). [Illustration: Fig. 108.--Diagram to illustrate the formation of the Germinal Layers. _E_, epiblast; _M_, mesoblast.] As to the origin of the mesoblast most observers have found[178] that a long groove (the _germinal groove_) appears in the middle line of the ventral plate (fig. 108), which bulges into the yolk, gradually detaches itself from the epiblast, and completes itself into a tube. The lumen of this tube soon becomes filled with cells, and the solid cellular mass thus formed divides into two longitudinal tracts, which lie right and left of the middle line of the ventral plate beneath the epiblast, and are known as the _mesoblastic bands_. In the Cockroach I was able to satisfy myself that in this Insect also, the mesoblast, in all probability, arises by the formation and closure of a similar groove of the epiblast. _M_ (fig. 108) represents the stage in which the lumen of the groove has disappeared, and the mesoblast forms a solid cellular mass. [178] Kowalewsky in _Hydrophilus_, Graber in _Musca_ and _Lina_, Patten in _Phryganidæ_, myself in _Meloe_, &c.