. Darwin, and after Darwin. An exposition of the Darwinian theory and a discussion of post-Darwinian questions. Evolution; Heredity. Embryology. IZI of the evidences of phyletic (or ancestral) evolution, which are so abundantly furnished by all the subse- quent phases of ontogenetic (or individual) evolution. The multicellular body which is formed by the series of segmentations above described is at first a sphere of cells (Fig. 40). Soon, however, a watery fluid gathers in the centre, and progressively pushes the cells towards the circumference, until they there. Fig. 41.—Formation of the gas
. Darwin, and after Darwin. An exposition of the Darwinian theory and a discussion of post-Darwinian questions. Evolution; Heredity. Embryology. IZI of the evidences of phyletic (or ancestral) evolution, which are so abundantly furnished by all the subse- quent phases of ontogenetic (or individual) evolution. The multicellular body which is formed by the series of segmentations above described is at first a sphere of cells (Fig. 40). Soon, however, a watery fluid gathers in the centre, and progressively pushes the cells towards the circumference, until they there. Fig. 41.—Formation of the gastrula of ^;«//^?(7XMj. (After Kowalevsky. A, wall of the ovum, composed of a single layer of cells ; B, a stnge in the process of gastrulation; C, completion of the process; S, original or segmentation cavity ef ovum ; al, alimentary cavity of gast] ula ; ect, outer layer of cells ; ent, inner layer of cells ; b, orifice, constituting the mouth in permanent forms. constitute a single layer. The ovum, therefore, is now in the form of a hollow sphere containing fluid, con- fined within a continuous wall of cells (Fig. 41 A). The next thing that happens is a pitting in of one portion ot the sphere (B). The pit becomes deeper and deeper, until there is a complete invagination of this part of the sphere—the cellswhich constitute it being progressively. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Romanes, George John, 1848-1894. Chicago The Open Court Pub. Co
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