PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 633 



the body in the same individual, so that only a general description 

 is necessary The most anterior group on the left side is cut in 

 the same section with the anterior wall of the ear. It consists of 

 six to eight cells lying close against the dorsal wall of the inferior 

 jugular vein, a small group on the right side (the most anterior of 

 this side) occur some distance back of this (about 60 micra), the 

 cells of which are pressed against the latero-ventral surface of the 

 sternohyoideus muscle of this region. From this point caudad 

 for 350 to 80 micra on either side, various other thyreoid follicles 

 occur, the exact details of the positions are omitted here. 



These developing follicles in this stage are also variable in form. 

 Some are globular or elliptical, with a well defined margin of 

 outer cells and have a lumen, while others are very irregular in 

 outline, and are solid masses of cells with no cavity. The first of 

 these types usually occur as isolated ollicles, lying singly in the 

 supporting tissue, while the others occur in regions of greater con- 

 gestion and crowding. So far as I am able to say, there is no 

 enveloping membrane (membrane propria) in this stage, either 

 surrounding the individual follicles, or enveloping collectively 

 the follicles to form the gland. 



F. Amblystoma larvae, 19 mm. long. In the 19 mm. stage, and 

 all subsequent larval stages studied, the thyreoid follicles are so 

 massed as to forma well defined structure on either side of the body, 

 the definitive thyreoid gland, which changes but slightly in its 

 position. The follicles here, as in the 15 mm. larvae, are irregular 

 in size, form and relative distribution about the wall of the inferior 

 jugular vein, and extend in a longitudinal direction a little more 

 than 400 micra on each side. As before, the most anterior fol- 

 licles are cut in section with the anterior wall of the ear, the most 

 caudal occur about 80 micra in front of the anterior wall of he 

 pericardium. Typically, the follicles in this stage are relatively 

 small, spherical or slightly ovate bodies, varying between 19 and 

 36 micra in major diameter, and showing in section, a layer of 

 cuboidal cells about the outer margin, enclosing a conspicuous 

 cavity, which contains a fluid of some sort but which as yet, does 

 not have the staining properties of a colloid. In some cases a 



