150 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



ectoderm pushing its way inwards. It extends inwards simply by 

 a process of delamination from the yolky " endoderm." 



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, p.t), 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 



