626 FRANCIS M. BALDWIN 



contents below ; and the lateral growth of the walls of the pharynx 

 to form the pharyngeal pouches; the pharyngeal cavity has been 

 greatly modified, being depressed dorso-ventrally. Below, and 

 to the sides of this pharyngeal tube, in the region between the 

 thickened oral plate (the mouth being still closed) and the anterior 

 wall of the pericardial chamber, numerous accumulations of 

 mesodermal cells occur, which have a considerable effect upon 

 the position of the thyreoid anlage in this and subsequent stages. 

 Figure 45 represents a reconstructed portion of the thyreoid 

 anlage in this stage, and shows its relation to the pharyngeal 

 entoderm, and the accumulations of mesodermal cells which are to 

 form the copula and the cartilages of the branchial apparatus. 



The thyreoid anlage is now a solid, cylindrical rod of cells 

 {tr., fig. 45) of considerable length (about 100 micra long) con- 

 nected anteriorly with the cells of the medial ventral floor of the 

 pharynx. Its distal end extends in a horizontal direction toward 

 the anterior wall of the pericardial chamber, but stops short of the 

 latter by about 40 micra. The fact that the thyreoid outgrowth 

 in Amblystoma does not reach the pericardial wall in this, and in 

 later stages, is a point of difference in its development from that 

 recorded by Miss Piatt in Necturus. In describing the thyreoid 

 outgrowth in a 13 mm. larva, which from her figure 6 (p. 562), cor- 

 responds closely to the stage here under discussion, she says, 

 ''From the floor of the alimentary canal, the thyreoid outgrowth 

 extends backward as a solid bar of tissue to the anterior wall of the 

 pericardium with which its posterior cells come into intimate 

 contact." This fact she further emphasizes in her conclusions, 

 "it remains in contiguity with the pericardium as long as its 

 union with the floor of the branchial cavity is retained." In 

 none of the developmental stages of Amblystoma have I seen any 

 intimate relation of the thyreiod anlage to the anterior wall of 

 the pericardium. 



Figure 4 is a medial sagittal section through the thyreoid 

 anlage (tr) in this stage. At the stomodeum (st), the entoderm 

 touches the ectoderm and forms an oral plate of considerable 

 thickness where later the mouth is to break through. In the 



