EARLY DEVELOPMENT OF THE COMMON NEWT. 473 



complete is their fusion. The external layer is never inter- 

 rupted. Fig. 14 presents a rounded thickening of the fused 

 mass, which is the next step in development. 



The latest stage we have (somewhat later than m) shows 

 the gland separated from the cpiblast (PI. XXI^ iig. 15) which 

 is now continuous across the middle line, but still connected 

 with the ventral wall of the oesophagus by a cord of cells. 

 The thyroid is now a solid cylindrical rod of considerable 

 length, ending posteriorly near the ventral aorta ; the section 

 shows an aortic arch (1 aa) cut through longitudinally. The 

 gland consists of an outer or cortical layer of columnar cells 

 arranged radially, and an inner small kernel of rounded 

 cells. As yet there is no trace of a lumen, or any division 

 into lobules. Further than this we have not been able to 

 follow its development, but have no reason to suppose that 

 it presents any great peculiarities. 



On the whole the thyroid body of the Newt corresponds 

 quite closely in position and mode of development to the 

 same body in the Elasmobranch Fishes ; but there are some 

 points of difference to which we should like to call par- 

 ticular attention. (1.) In the latter the diverticulum of the 

 hypoblast is hollow in front and solid behind at first, and 

 only subsequently becomes solid throughout^ while in Triton 

 we have not been able to discover any stage which shows a 

 hollow outgrowth. The solidity, however, does not occur 

 from any confused mass of cells, but from the fact that the 

 two sides of the diverticulum are pressed closely together 

 (PI. XXI, figs. 12 and 18). Of course it is very possible that 

 we have missed a stage in which the outgrowth was hollow ; 

 but if that is the case that condition must be a very tran- 

 sitory one. The difference is only one of detail in any case. 

 (2.) Of much more importance is the fact that in the Elas- 

 mobranchs there is never found any indication of continuity 

 between the hypoblast and epiblast, which at this period is 

 still single layered. But the diverticulum is pressed very 

 closely against the epiblast, presenting just the appearance 

 of the first visceral cleft which does not perforate the skin.^ 

 (We do not wish to intimate by this comparison an opinion 

 that the thyroid is a modified visceral cleft, because all diver- 

 ticula from the throat to the external skin must look more 

 or less alike.) 



The account given by Dr. Gotte^ of the development of 

 the thyroid in Bonibinator is still more like our account 

 than is that given by Mr. Balfour of the Elasmobranchs. 



1 Balfour, loc. cit., Plate XIV, fig. 5«, p. 223-5. 

 - Loc. pit., p. 6G7. 



