150 EMBRYOLOGY OF THE LOWER VERTEBRATES OIL 



ectoderm pushing its way in wan Is. It extends inwards simply by 

 a process of delamination from the yolky " endoderm." 



The real lesson to be learnt from these cases is that the 

 characters of one germ-layer are liable to spread over its boundary 

 into territory belong n.u r 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 



C. 



1 i . 84. Sagittal sections illustrating the development of the tongue in UroiU-li->. 



A and B, Triton ; C, Salmnandra (alter Kullius, 11*01) ; </./, gland field ; Al, maudibular airh ; 

 l>.t, primary tonj;-"' 1 . 



giving any definitive proof as to which of the two cell-layers that 

 organ belongs to. 



THK TONGUE. The tonkin- is a portion of the buccal floor which 

 becomes demarcated oil' 1mm the iv>l by a. split formed by a down- 

 growlh 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. Her.- then- develops first a primary tongue, 

 ensheathing the anterior and ventral portion of the hyoid arch 

 84, ;;.), \\hieh becomes marked off, except at its hinder end. by 

 a deep 'jroove in the Hour of the mouth. 



A hoi aped thiekenini: o| the buci-.nl epithelium no\\ 



