28 THYMUS AND PARATHYROID GLANDS 



and of the tetanic larva (right vertical row) are drawn at the same 

 magnification.^ 



Still more significant are the differences in structure. In the 

 pretetanic larva? the thymus glands represent small accumulations of 

 purely epithelial cells. They do not show any kind of differentiation. 

 Their epithelial character is indicated also by the presence of consid- 

 erable quantities of pigment granules such as are found in the epi- 

 thelium of the pharyngeal pouches from which the glands have 

 separated. In most of the glands there are no mitoses to be found at 

 all, while in some there are one to two mitoses present. No mem- 

 brana propria is formed. 



Compared with the thymus of the pretetanic larvae, in which none 

 of the characters of a true thymus has developed, five of the glands 

 of the tetanic larva show some of the aspects of a real thymus in as far 

 as they contain elements which are characteristic of that organ. 

 Again no membrana propria is formed, but the glands consist of a 

 variety of cells. Leaving aside the question as to whether or not the 

 newly developed cell elements are of mesenchymal origin,^ it is 

 sufficient to notice that several kinds of cells now constitute these 

 glands; two of them are recognized clearly as the main elements mak- 

 ing up the fully developed thymus of an Amby stoma opacum larva. 



^ The first and second pairs of the thymus glands have disappeared already in 

 this stage from both the pretetanic and tetanic larvae, and only the third, fourth, 

 and fifth pairs are present. According to Maximow and to Baldwin the first and 

 second pairs are found absent already in larvae of from 1 1.5 to 13.0 mm. in length. 



* Concerning the development of the amphibian thymus glands and the origin 

 of their cell elements see the papers by Maximow, by Dustin, and by Baldwin. 

 It should be mentioned that the development of the thymus glands in Ambys- 

 toma opacum was found by the writer to correspond in the main with the descrip- 

 tion as given by Maximow and Baldwin for Ambystoma tigrinum and Amby stoma 

 maculatmn respectively. The size of the larvae, however, corresponding to the 

 various developmental stages of the glands was found to be much more variable 

 and in general larger. In a normal stock animal of Amby sterna opacum, for in- 

 stance, the right gland of the third pair was found to be still connected by ; n 

 epithelial stalk with the brancho-pharyngeal epithelium and of purely epithelial 

 character, though the larva has a length of over 37 mm., while Maximow and 

 Baldwin found the glands fully separated in larvae of from 10 to 12 mm.; Maximow 

 saw the commencement of mesenchymal immigration in larvae of about 10 mm. 

 in length, and Baldwin in larvae of 19 to 20 mm in length. 



