472 W. B. SCOTT AND HENRY F. OSBORN. 



head forms an apparent exception. Not far behind the 

 auditory involution, there are visible at the end of Period K 

 a few longitudinal muscles, forming about three or four mus- 

 cle plates, the ventral part of which is wanting. I have not 

 the means of deciding whether they properly belong to the 

 head or may not be a part of the trunk system of muscles 

 which has to a certain extent overlapped the back of the 

 head, but am inclined to accept the latter view/^ The 

 appearances here described are very much like those to be 

 seen in Triton, and Ave are not in a position to pronounce 

 any more decided judgment upon them, than upon those of 

 the Elasmobranchs ; but taking into consideration Gotte's 

 figures we are rather inclined to consider them the axial 

 segments of which the plate containing the head cavity is 

 the lateral part. The chief differences between the two 

 types of Amphibians lie in the cavities themselves, and 

 the number of segments in the anterior part of the 

 head. 



Oar researches do not, we regret to say, throw much 

 new light upon that difficult morphological problem, the 

 segmentation of the head. It is interesting to find that 

 as in the Elasmobranchs there is one praj-oral segment, as 

 might be expected to be the case if the head cavities afford 

 any trustworthy guide to the number of head segments. Of 

 course the number of postoral cavities is less than in the 

 Elasmobranchs owing to the fewer gill clefts, but this is a 

 feature which does not affect the question at issue. 



Of whatever value these facts in the development of the 

 Newt are considered, we think that they favour the views 

 expressed by Mr. Balfour in p. 216 of his book. For these 

 head cavities, if of morphological importance, might be 

 anticipated to be fairly constant in character. 



The Thyroid Body. 



PI. XXI, fig. 12, represents the earliest condition of the thy- 

 roid body which has fallen under our observation. In it we 

 see that in the region of the mandibular arch there is a 

 solid outgrowth of cells from the ventral wall of the alimen- 

 tary cavity which has reached the inner layer of the epiblast. 

 The latter has at the point of contact risen up slightly from 

 the external layer leaving a small triangular space between 

 them. In the next stage (fig. 13), the inner layer of epi- 

 blast has coalesced with the hypoblastic outgrowth and is 

 discontinuous across the middle line. It is now difficult to 

 determine where one layer begins and the other ends^ so 



