. Text-book of embryology. Embryology. 150 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. ectoderm pushing its way inwards. It extends inwards simply by a process of delamination from the yolky " ; The real lesson to be learnt from these oases is that the characters of one germ-layer are liable to spread over its boundary into territory belonging to another layer or, in other words, that the territories of the various layers are liable to be separated by an indefinite debatable zone rather than by a mathematically sharp line. It follows that the apparent position of an organ-rudime
. Text-book of embryology. Embryology. 150 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. ectoderm pushing its way inwards. It extends inwards simply by a process of delamination from the yolky " ; The real lesson to be learnt from these oases is that the characters of one germ-layer are liable to spread over its boundary into territory belonging to another layer or, in other words, that the territories of the various layers are liable to be separated by an indefinite debatable zone rather than by a mathematically sharp line. It follows that the apparent position of an organ-rudiment in relation to such a boundary is not necessarily to be taken as. Fig. 84.—Sagittal sections illustrating the development of the tongue in Urodeles. A and B, Triton ; C, SalcCmandra (after Kallius, 1901) ; </./, gland field ; Jlf, mandibular arch ; 2?.t, primary tongue. giving any definitive proof as to which of the two cell-layers that organ belongs to. The Tongue.—The tougue is a portion of the buccal floor which becomes demarcated off from the rest by a split formed by a down- growth of the lining epithelium of the mouth. Its mode of develop- ment is well illustrated by what happens in Urodele Amphibians as described by Kallius. Here there develops first a primary tongue, ensheathing the anterior and ventral portion of the hyoid arch (Fig. 84, ), which becomes marked off, except at its hinder end, by a deep groove in the floor of the mouth. A horseshoe-shaped thickening of the buccal epithelium now. 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-; Heape, Walter, 1855-1929. London : Macmillan
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