Although Hydra itself constitutes the simplest type of Hydrozoon, its development, which has been fully investigated by Kleinenberg (No. 161), is in some respects a little exceptional. The segmentation is regular, but a segmentation cavity is not formed. The peripheral layer of cells gradually becomes converted into a chitinous membrane, which is perhaps homologous with the perisarc of marine forms. Between the membrane and the germ a second pellicle makes its appearance. The above changes require about four days for their completion, but there next sets in a period of relative quiescence which lasts for some 6-8 weeks. During this period the remaining development is completed. The cells of the germ first fuse together. In the interior of the protoplasm a clear excentric space arises, which gradually extends itself and forms the rudiment of the gastric cavity. The outer shell in the meantime becomes less firm, and is finally burst and thrown off, owing to the expansion of the embryo within. The outermost layer of the protoplasm becomes, relatively to the inner layer, clear and transparent, and there thus arises an indication of a division of the walls of the archenteric cavity into two zones, or layers. These layers, which form the epiblast and hypoblast, are definitely established on the appearance of cells with contractile tails[76] in the clear outer zone, between which the interstitial epiblast cells subsequently arise. [76] These cells are the so-called nerve-muscle cells. Their nature is discussed in the second part of this work. The embryo, still forming a closed double-walled sack, elongates itself, and at one pole its wall becomes very thin. And at this point a rupture takes place which gives rise to the mouth. Simultaneously with the mouth the tentacles become formed as hollow processes, according to Mereschkowsky two being formed first and subsequently the others in pairs. Very shortly afterwards the hitherto uniform hypoblast becomes divided up into distinct cells. The thin inner pellicle which persists after the rupture of the outer membrane becomes in the meantime absorbed. With these changes the embryo practically acquires the characters of the adult. Trachymedusæ. Amongst the Trachymedusæ, which as has now been satisfactorily established develop directly without alternations of generations, the embryology of species both of the Geryonidæ and the Æginidæ has been studied. In all the types so far investigated the hypoblast is formed by delamination, and there is a more or less well-marked planula stage. [FIG. 70. DIAGRAMMATIC FIGURE SHEWING THE DELAMINATION OF THE OVUM OF GERYONIA. (Copied from Fol.) _cs._ segmentation cavity; _a._ endoplasm; _b._ ectoplasm. The dotted lines shew the course of the next planes of division.]