474 W. B. SCOTT AND HENllY F. OSBORN. 



In Bombinator the thyroid "is formed from a pit of the 

 hypoblast, which persists as the remains of an early depres- 

 sion of the hypoblast behind the mandibular arch, i^roduced 

 hy a fusion of the epiblast and hypohlasf (Taf. vii, fig. 127- 

 130, and Taf. xiii,xv, xvi,figs. 292 and 293.) At first it is 

 connected anteriorly with the median division line which 

 bisects that arch ; after the disappearance of this the rudi- 

 ment of the thyroid appears as a funnel-shaped diverticulum 

 of the hypoblast and is free below. The fusion between the two 

 layers, which in Triton persists for a considerable period and 

 is seen throughout the length of the gland, here is confined 

 to the anterior end, and remains only a short time. 



W. Miiller, in his account of the development of the thy- 

 roid body^ in Rana temporaria, does not give any figures or 

 descriptions leading us to suppose that he has observed this 

 continuity of the layers. 



We must confess that we ourselves are very much puzzled 

 by the fusion of the epiblast and hypoblast at this point, and 

 are unable to give any morphological explanation of its 

 meaning. Is it not just possible that it may represent some 

 shifting in the position of the mouth ? but if so, we shall be 

 obliged to abandon, for this form at least, the homology of 

 the thyroid body with the endostyle of the Ascidians. We 

 mention it with the hope of directing the attention of some 

 morphologist, who will clear the matter up, to this curious 

 and unexplained feature. 



It may be of use to give a brief summary of the points 

 which we have endeavoured to establish in this paper, before 

 passing on to consider to what general conclusions these 

 points lead us, if established. 



1. As to external features , we have failed to find in Triton 

 the suckers and horny teeth with which the Batrachian 

 larva is furnished. 



2. Segmentation proceeds in a manner much like that of 

 the Frog, but the roof of the segmentation cavity is from the 

 very first only one cell thick. 



3. An unsymmetrical invaxfination^ like that of the Frog 

 and Lamprey, takes place, giving rise to one layer in the 

 middle line, the hypoblast, and two at the sides, hypoblast 

 and mesoblast. The invagination mesoblast is supplemented 

 by other cells, which split off* from the yolk hypoblast. These 

 two lateral and disconnected masses of mesoblast are, we 

 consider, the homologues of the paired hypoblastic divers 

 ticula in Amphioxus. 



4. The epiblast is at first composed of a single layer of 



' Jenaische, ' Zeitschrift,' 1871, pp. 435-439. 



