[14] In most vertebrated animals this process of gastrulation has been more or less superseded by another, which is called delamination; but it scarcely seems necessary for our present purposes to describe the latter. For not only does it eventually lead to the same result as gastrulation--i. e. the converting of the ovum into a double-walled sac,--but there is good evidence among the lower Vertebrata of its being preceded by gastrulation; so that, even as to the higher Vertebrata, embryologists are pretty well agreed that delamination has been but a later development of, or possibly improvement upon, gastrulation. [Illustration: FIG. 41.--Formation of the gastrula of _Amphioxus_. (After Kowalevsky.) A, wall of the ovum, composed of a single layer of cells; B, a stage in the process of gastrulation; C, completion of the process; S, original or segmentation cavity of ovum; _al_, alimentary cavity of gastrula; _ect_, outer layer of cells; _ent_, inner layer of cells; _b_, orifice, constituting the mouth in permanent forms.] [Illustration: FIG. 42.--Gastrulation. A, Gastrula of a Zoophyte (_Gastrophysema_). (After Häckel.) B, Gastrula of a Worm (_Sagitta_). (After Kowalevsky.) C, Gastrula of an Echinoderm (_Uraster_). (After A. Agassiz.) D, Gastrula of an Arthropod (_Nauplius_). (After Häckel.) E, Gastrula of a Mollusk (_Limnæus_). (After Rabl.) F, Gastrula of a Vertebrate (_Amphioxus_). (After Kowalevsky.) In all, _d_, indicates the intestinal cavity; _o_, the primitive mouth; _s_, the cleavage-cavity; _i_, the endoderm, or intestinal layer; _e_, the ectoderm or skin-layer.] [Illustration: FIG. 43.--Gastrula of a Chalk Sponge. (After Häckel.) A, External view. B, Longitudinal section. _g_, digestive cavities; _o_, mouth; _i_, endoderm; _e_, ectoderm.] [Illustration: FIG. 44.--_Prophysema primordiale_, an extant gastræa-form. (After Häckel.) (A). External view of the whole animal, attached by its foot to seaweed. (B). Longitudinal section of the same. The digestive cavity (_d_) opens at its upper end in the mouth (_m_). Among the cells of the endoderm (_g_) lie amoeboid egg-cells of large size (_e_). The ectoderm (_h_) is encrusted with grains of sand, above the sponge spicules.]