112 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



19 and 40mm. larv^al stages; of the thymus tilaiul in the 19 and 

 35mm. stage as well as the early, late and adult stages; of the 

 postbranchial body in the 19, 40mm., and early transforming 

 stages; of the epithelial bodies in the late transforming and 

 adult stages (these appear only at the time of metamorphosis 

 and persist in the adult) ; and of the carotid gland in the late 

 transforming and adult stages, (this gland appears during the 

 time of metamorpliosis also, but as pointed out later is not a 

 tnie pharyngeal derivative). 



Since the entire results of my .study have been published in 

 the last issue of the Journal of Morphology (March, 1918), Vol. 

 30, No. 2, it is necessary here only to point out in a summary 

 way some of the more important conclusions. 



The anlagen of the thymus bodies are recoo'nized in Ambly- 

 stoma lai*A'fe 8mm. long, as epithelial thickenings at the caudo- 

 dorsal extremity of five pharyngeal pouches (counting the 

 hyomandibular pouch as the first). These thickenings soon be- 

 come solid epithelial bodies, which lose their early connection 

 with the pharyngeal entoderm, and lie in the connective tissue, 

 dorsal to their respective pouches. These bear certain relation- 

 ships in position to the ganglia of the cranial nerves and the ear. 



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. 



Of the five primitive epithelial thymus bodies, the first two 

 degenerate early; this degeneration takes place with some irreg- 

 ularity in different individuals since in some larvae, the second 

 body had disappeared in the 11mm. stage, while the first still 

 persisted as a very slender stalk of cells. Usually, however, 

 l)oth bodies had disappeared by the 15mm. stage. 



The three remaining bodies are, at first, small, spherical or 

 slightly oval structures, of about equal size (in 19nnn. larva?, 

 measuring 90x75 micra, 76x60 miera and 90x120 micra respect- 

 ively) but during the later larval stages they increase gradually 

 in size and elongate somewhat so that in larvae 45mm. long (the 

 stage before transformation), they have the dimensions of 

 220.\164, 180x240, and 285.\.S50 micra respeetively. 



The three bodies during the larval stages, lie close behind one 

 another and caudolateral to the posterior wall of the ear. The 



