PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 635 



length on either side, the anterior end occurring in sections with 

 the anterior wall of the ear, the caudal region extending to within 

 60 or 75 micra of the anterior wall of the pericardium. In addi- 

 tion to this increase in length, the gland is now enveloped in a very 

 thin connective tissue sheath, which surrounds not only the fol- 

 licles and lymph and vascular spaces, but also includes the large 

 trunk of the inferior jugular vein for a considerable distance in its 

 course through this region. The inferior jugular vein enters this 

 sheath, but in this and later stages, it does not break up into a 

 'rete mirabile.' The external carotid artery, which courses 

 lateral to the gland, is not enveloped in the connective tissue 

 sheath of the gland and has no intimate connection with the 

 thyreoid in this stage. The lumina of the follicles are now larger 

 in comparison than before, but as yet there is no colloid. 



/. Amblystoma larvae, 39 mm. long. In the 39 mm. larva, the 

 thyreoid gland is a well defined ovate, somewhat flattened body, 

 a little less than a millimeter long, lying dorso-medial to the in- 

 ferior jugular vein on either side of the body. The 'connective 

 tissue envelope is here more pronounced, and a few of the fol- 

 licles contain colloid. Figure 47 represents a wax reconstruction 

 of the gland and the cartilages of the branchial apparatus in this 

 stage. The follicles are rather large throughout, and are more 

 numerous in the central and caudal region. 



J. Amblystoma larvae, 1^5 mm. long. In the 45 mm. stage (the 

 last to be described before transformation), the gland is about a 

 millimeter long. Nearly all of the follicles are now filled with 

 colloid. In a section through the central region of the gland, (fig. 

 31), eleven follicles are cut; these almost completely surround the 

 inferior jugular vein, which in this region is within the connective 

 tissue sheath. The follicles are separated from one another by 

 vascular and lymph spaces {v.v.). Nearly all the foUicles in this 

 section show an outer lining of cuboidal cells with spacious lumina 

 within. 



K. Transforming Amhlystomae. In the early stages of trans- 

 formation, the gland is noticeably shorter, and, with the greatly 

 increased size of the surrounding muscles (genio- and sterno- 



