. Text-book of embryology. Embryology. GASTKULATION 33 The fact that the yolk portion of the blastula consists not of a single layer of cells as in Amphioxus but of a solid bulky mass forming a large proportion of the whole blastula, renders it physically impossible for the yolk hemisphere to be involuted bodily into the interior of the apical hemisphere. As a ^ | consequence we find in Polypterus that the' process of in volution is replaced to a greater extent than in Amphioxus by overgrowth, the gastrular lip ing over the mass of yolk-cells as seen in Kg. 19, C, D and E. As this process of o
. Text-book of embryology. Embryology. GASTKULATION 33 The fact that the yolk portion of the blastula consists not of a single layer of cells as in Amphioxus but of a solid bulky mass forming a large proportion of the whole blastula, renders it physically impossible for the yolk hemisphere to be involuted bodily into the interior of the apical hemisphere. As a ^ | consequence we find in Polypterus that the' process of in volution is replaced to a greater extent than in Amphioxus by overgrowth, the gastrular lip ing over the mass of yolk-cells as seen in Kg. 19, C, D and E. As this process of overgrowth con tinues the project ing yolk-plug—the mass of yolk - cells not yet enclosed —gradually dimin ishes in size and eventually disap pears c6mpletely in the now narrow blastopore. As yet material is not avail able to show defin- itely whether the overgrowth is more active in what corre sponds to the an terior portion of the gastrular lip of Amphioxus but the probability is in favour of this being the case and the figures are orien-. Fig. 19.—Illustrating process ofgastrulation in Polypterus. (Figs. A, C, E, F after drawings by Budgett.) , gastrular lip; , yolk-plug. The egg is in each case viewed from the left side and has the dorsal side above. The original apical pole is directed downwards and towards the left side of the page as in tated On the aSSUmp- the preceding figure of Amphioms. tion that this is so. The two features to be specially noted in the gastrulation of Polypterus as compared with that of Amphioxus are (1) the accentuation of the process of overgrowth and the reduction of the process of involution and (2) the tendency, in early stages at least, VOL. II D. 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 MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869
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Keywords: ., bookcentury1900, bookdecade1910, bookpubli, booksubjectembryology
