PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 667 



are anatomically separate and distinct in the lower vertebrata 

 but which come into very intimate anatomical and physiological 

 relationships with each other in the mammalia." Their func- 

 tions, so far as they are known, are very different. 



IV. GENERAL SUMMARY 



1 . The anlagen of the thymus bodies are recognized in Ambly- 

 stoma larvae 8 mm. long, as epithelial thickenings at the caudal 

 dorsal extremity of five pharyngeal pouches (counting the hyo- 

 mandibular pouch as the first). These thickenings soon become 

 solid epithelial bodies, which lose their early connection with the 

 pharyngeal entoderm, and lie in the connective tissue dorsal to 

 their respective pouches. 



2. The darkly staining cells which are scattered or grouped 

 about in the region close to the thymus bodies are regarded as 

 mesenchymatous in origin and are not ectodermal contributions 

 to the thymus bodies. 



3. Of the five primitive epithelial thymus bodies, the first two 

 degenerate early; this degeneration takes place with some irregu- 

 larity in different individuals. 



4. The three remaining bodies are, at first, small, structures of 

 about equal size; during the later larval stages they increase 

 gradually in size and become more elongate. 



5. The three bodies persistent during the larval stages, lie close 

 behind one another and caudo-lateral to the posterior wall of thp 

 ear. The third and fourth are usually close together, while 

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



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

 structure (probably formed by the union of the three larval thy- 

 mus bodies) lying in the side of the neck, caudad and a little dor- 

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

 tissue richly supplied with vascular vessels, and is innervated by 

 rami of the glossopharyngeal and vagus nerves. 



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

 mm. long, as a very shallow cup-like depression in the medial floor 

 of the pharynx in the region of the hyomandibular pouch be- 



