PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 113 



third and fourth bodies are usually close together, while between 

 the fourth and fifth bodies there is a much larger interval. 



In the adult, the thymus gland is a three-lobed flattened struc- 

 ture (probably formed by the union of the three larval thymus 

 bodies) lying in the side of the neck, caudad and a little dorsal 

 to the angle of the jaw. It is surrounded by loose connective 

 tissue richly supplied with vascular vessels, and innervated by 

 rami of the glossopharyngeal and vagus nerves. 



The anlage of the thyreoid gland is recognized in larvae 5mm. 

 long, as a very shallow cuplike depression in the medial tloor 

 of the pharnyx in the region of the hyomandibular pouch (not 

 the second branchial pouch as Maurer '88 states for Triton) 

 between the thickened oral plate, and the anterior limits of the 

 pericardium. 



As development proceeds, this cup-shaped anlage proliferates 

 cells from its ventral surface so that in 8mm. larvae a solid 

 elongated stalk €611 reaches in a caudal direction toward the 

 anterior wall of the pericardium a distance of about 100 micra, 

 but it does not unite with the anterior wall in any way (differ- 

 ing in this respect from the similar formed structure described 

 by Miss Piatt in Necturus). A little later (9.5mm. larv^) the 

 distal end of the solid stalk begins to divide into right and left 

 halves, and this continues with some rapidity so that in 10mm. 

 larva? it is completed, and in addition, the anterior connection 

 with the pharynx is lost and the two halves lie lateral to the 

 geniohyoideus muscle of either side. 



I find no evidence that the cells which formed the connecting 

 stalk of the two halves of the thyreoid, or those which formed 

 the connecting stalk persist to form accessory thyreoids as was 

 described by Maurer in Triton. 



After becoming completely divided into right and left halves, 

 each half loses its solid and compact condition. The epithelial 

 cells scatter loosely into the connective tissue and arrange them- 

 selves irregularly about the dorsal wall of the inferior jugular 

 vein of either side (larvte 13mm. long) and from these loosely 

 scattered epithelial cells the follicles of the gland arise by mi- 

 totic division. 



Follicles first appear in larva? 15mm. long; they are either 



globular or elliptical, with a well defined outer layer of cuboidal 



cells enclosing a conspicuous cavity, probably containing a fluid 



of some sort but no colloid. They vary greatly in size, but, as 



8 



