A. Blastosphere stage with hypoblast spheres becoming budded off into central cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric cavity. _ep._ epiblast; _hy._ hypoblast; _al._ gastric cavity.] Delamination and invagination are undoubtedly the two most frequent modes in which the layers are differentiated, but there are in addition several others. In the first place the whole of the Tracheata (with the apparent exception of the Scorpion) develop, so far as is known, on a plan peculiar to them, which approaches delamination. This consists in the appearance of a superficial layer of cells enclosing a central yolk mass, which corresponds to the hypoblast (figs. 204 and 214). This mode of development might be classed under delamination, were it not for the fact that the early development of many Crustacea is almost the same, but is subsequently followed by an invagination (fig. 208), which apparently corresponds to the normal invagination of other types. There are strong grounds for thinking that the tracheate type of formation of the epiblast and hypoblast is a _secondary modification of an invaginate type_ (_vide_ Vol. II. p. 457). [FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE EMBRYO OF GERYONIA. (After Fol.) A. Stage at the commencement of the delamination; the dotted lines _x_ shew the course of the next planes of division. B. Stage at the close of the delamination. _cs._ segmentation cavity; _a._ endoplasm; _b._ ectoplasm; _ep._ epiblast; _hy._ hypoblast.] [FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER. (After Metschnikoff.) In A the ovum is divided into a number of separate segments. In B a number of small cells have appeared (_bl_) which form a blastoderm enveloping the large yolk-spheres. In C the blastoderm has become divided into two layers.] The type of some Turbellaria (_Stylochopsis ponticus_) and that of Nephelis amongst the Discophora is not capable of being reduced to the invaginate type. The development of almost all the parasitic groups, _i.e._ the Trematoda, the Cestoda, the Acanthocephala, and the Linguatulida, and also of the Tardigrada, Pycnogonida, and other minor groups, is too imperfectly known to be classed with either the delaminate or invaginate types. It will, I think, be conceded on all sides that, if any of the ontogenetic processes by which a gastrula form is reached are repetitions of the process by which a simple two-layered gastrula was actually evolved from a compound Protozoon, these processes are most probably of the nature either of invagination or of delamination. The much disputed questions which have been raised about the gastrula and planula theories, originally put forward by Haeckel and Lankester, resolve themselves then into the simple question, whether any, and if so which, of the ontogenetic processes by which the gastrula is formed are repetitions of the phylogenetic origin of the gastrula.