I regard the gastrula as the most important and significant embryonic form in the animal world. In all real animals (that is, excluding the unicellular protists) the segmentation of the ovum produces either a pure, primitive, palingenetic gastrula (Figure 1.29 I, K) or an equally instructive cenogenetic form, which has been developed in time from the first, and can be directly reduced to it. It is certainly a fact of the greatest interest and instructiveness that animals of the most different stems--vertebrates and tunicates, molluscs and articulates, echinoderms and annelids, cnidaria and sponges--proceed from one and the same embryonic form. In illustration I give a few pure gastrula forms from various groups of animals (Figures 1.30 to 1.35, explanation given below each). (FIGURES 1.30 TO 1.35. In each figure d is the primitive-gut cavity, o primitive mouth, s segmentation-cavity, i entoderm (gut-layer), e ectoderm (skin layer). FIGURE 1.30. (A) Gastrula of a very simple primitive-gut animal or gastraead (gastrophysema). (Haeckel.) FIGURE 1.31. (B) Gastrula of a worm (Sagitta). (From Kowalevsky.) FIGURE 1.32. (C) Gastrula of an echinoderm (star-fish, Uraster), not completely folded in (depula). (From Alexander Agassiz.) FIGURE 1.33. (D) Gastrula of an arthropod (primitive crab, Nauplius) (as 32). FIGURE 1.34. (E) Gastrula of a mollusc (pond-snail, Linnaeus). (From Karl Rabl.) FIGURE 1.35. (F) Gastrula of a vertebrate (lancelet, Amphioxus). (From Kowalevsky.) (Front view.)) In view of this extraordinary significance of the gastrula, we must make a very careful study of its original structure. As a rule, the typical gastrula is very small, being invisible to the naked eye, or at the most only visible as a fine point under very favourable conditions, and measuring generally 1/500 to 1/250 of an inch (less frequently 1/50 inch, or even more) in diameter. In shape it is usually like a roundish drinking-cup. Sometimes it is rather oval, at other times more ellipsoid or spindle-shaped; in some cases it is half round, or even almost round, and in others lengthened out, or almost cylindrical.