244 ALICE JOHNSON AND LILIAN SHELDON. 



shape of the adult mouthy becoming first elongated trans- 

 versely, and then horseshoe shaped, with the concavity of the 

 horseshoe directed backwards. The consequence is that, in 

 transverse sections of late stages, the mouth appears to consist 

 of two lateral parts, which are the limbs of the horse-shoe. 

 We find that the pituitary body and stomodseum develope in 

 exactly the same way in the Frog as in the Newt. 



The pituitary body has been described as originating from a 

 solid ingrowth of epiblast in Teleosteans by Hoffmann (13), and 

 it seems to arise somewhat similarly in Lepidosteus (2) . Grotte 

 also describes the same method of development in Bombinator 

 (9). (See his figs. 127, 128, 250, 252, 292, 293, 298, and 305.) 



The Thyroid Body. 



From the hind end of the stomodseum proceeds a solid cord 

 of cells continuous along its dorsal border with the fore-gut 

 (figs. 9, 10, 11). This is the thyroid body. Later a groove 

 is continued into it from the fore-gut, and its hind part 

 becomes a tube by the folding over of the edges of the groove. 

 Subsequently the hind end is completely constricted off from 

 the gut. We have not followed its development further. 

 Scott and Osborn (19) described it as being formed from a 

 fusion of hypoblast and epiblast in the median ventral line. 

 We think that this fusion is the stomodseum, with which the 

 thyroid is continuous at its front end, and that the thyroid 

 itself is developed in a perfectly normal manner. 



Development of PERiPHEaAi. Nervous System. 



There is no trace of the peripheral nervous system until the 

 neural canal has completely closed and become separate from 

 the external epiblast. Fig. 15 represents a transverse section 

 through an embryo of a stage just before the closure, showing 

 the epiblast in close contact with the neural canal, with which 

 its two layers are of necessity continuous at this time. 



The appearance of the peripheral nervous system is preceded 

 by the formation of a neural ridge. In an embryo in which 

 this is first seen, the neural canal has lost all connection with 



