NOTES ON THE DEVELOPMENT OF THE NEWT. 25] 



clefts shows them to be of segmental value, since they are in 

 some cases situated one above each gill-cleft. At the same 

 time he is obliged to assume the existence of more than one 

 now aborted gill-cleft, in order to account for the number of 

 the sense-organs. If the proof of the segmental value of the 

 sense organs is to depend on the number of the gill-clefts, and 

 the number of the gill-clefts is in turn to depend on the 

 segmental value of the sense organs, it is difficult to discern 

 which is the basis of the argument. Malbranc (15) shows that 

 even in the embryo multiplication of the sense organs by 

 division may occur, so that the number of them seems to be 

 indefinitely variable ; and Mr. Beard himself has described such 

 a division in the case of the sense organs of the facial nerve. 

 It seems, therefore, that there was primitively only one such 

 sense organ in this case, and that one cannot depend on the 

 number of the sense organs at any but the very earliest stages, 

 if even then, as indicating segmentation. 



Summary of Observations. 



1. A solid post-anal gut is formed behind the blastopore (anus), 

 growing out into the tail, and fusing with the undifferentiated 

 tissues at its posterior end. The fusion of hypoblast and 

 epiblast in this region represents the neurenteric canal. 



2. In the Frog the post-anal gut is at first hollow, but after- 

 wards becomes solid. 



3. The stomodaeum and pituitary body are derived from a 

 solid ingrowth of the inner layer of the epiblast. The hind 

 part of this ingrowth fuses with the front wall of the fore-gut, 

 but the perforation to form the actual mouth does not appear 

 till after hatching. The pituitary body grows upwards as a 

 solid cord, and applies itself to the infundibulum in the ordinary 

 manner. 



4. From the hind border of the stomodseum proceeds a solid 

 rod of cells, which constitutes the thyroid body, and is de- 

 veloped from the cells of the middle ventral line of the fore- 

 gut. 



