There are various peculiar modifications of invagination which cannot be dealt with in detail. Invagination in one form or other occurs in some or all the members of the following groups: [FIG. 200. TRANSVERSE SECTION THROUGH THE OVUM OF EUAXES DURING AN EARLY STAGE OF DEVELOPMENT, TO SHEW THE NATURE OF EPIBOLIC INVAGINATION. (After Kowalevsky.) _ep._ epiblast; _ms._ mesoblastic band; _hy._ hypoblast.] The Dicyemidæ, Calcispongiæ (after the amphiblastula stage) and Silicispongiæ, Coelenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa, Brachiopoda, Chætopoda, Discophora, Gephyrea, Chætognatha, Nematelminthes, Crustacea, Echinodermata, and Chordata. The gastrula of the Crustacea is peculiar, as is also that of many of the Chordata (Reptilia, Aves, Mammalia), but there is every reason to suppose that the gastrulæ of these groups are simply modifications of the normal type. 2. Delamination. Three types of delamination may be distinguished: _a._ Delamination where the cells of a solid morula become divided into a superficial epiblast, and a central solid mass in which the digestive cavity is subsequently hollowed out (fig. 201). [FIG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM, TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After Metschnikoff.) A. Stage after the delamination; _ep._ epiblastic invagination to form pneumatocyst. B. Later stage after the formation of the gastric cavity in the solid hypoblast. _po._ polypite; _t._ tentacle; _pp._ pneumatocyst; _ep._ epiblast of pneumatocyst; _hy._ hypoblast surrounding pneumatocyst.] _b._ Delamination where the segmented ovum has the form of a blastosphere, the cells of which give rise by budding to scattered cells in the interior of the vesicle, which, though they may at first form a solid mass, finally arrange themselves in the form of a definite layer around a central digestive cavity (fig. 202). _c._ Delamination where the segmented ovum has the form of a blastosphere in the cells of which the protoplasm is differentiated into an inner and an outer part. By a subsequent process the inner parts of the cells become separated from the outer, and the walls of the blastosphere are so divided into two distinct layers (fig. 205). Although the third of these processes is usually regarded as the type of delamination, it does not, so far as I know, occur in nature, but is most nearly approached in Geryonia (fig. 203). The first type of delamination is found in the Ceratospongiæ, some Silicispongiæ (?), and in many Hydrozoa and Actinozoa, and in Nemertea and Nematelminthes (_Gordioidea?_). The second type occurs in many Porifera [_Calcispongiæ_ (_Ascetta_), _Myxospongiæ_], and in some Coelenterata, and Brachiopoda (_Thecidium_). [FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.)