PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 629 



Die Schilddriisenanlage wird von ihrem Mutterboden weiter entfernt, 

 diirch die Copula des Hyoidbogens. Ziigleich beginnt sie sich zu theilen, 

 so dass an fimften Tage die beiden Halften nur diirch einen diinnen 

 Isthmus zusammenhangen. 



The division of the thyreoid anlage described above leads to 

 the brief consideration of two points; the fate both of the cells 

 which formed the connection between it and the pharynx (the 

 duct of the other glands) and of those which connected the halves 

 before their separation. Miss Piatt ('96), in Necturus, found 

 cells from the anterior part of the ' broken outgrowth ' (thyreoid) 

 distributed upon the dorsal surface of the mylohyoideus muscle, 

 which were finally lost in the muscular tissue. She also found a 

 few scattered cells from the anterior part of this outgrowth, above 

 the level of the geniohyoideus muscle, the fate of which she 

 could not follow, but in her material they did not develop into 

 accessory thyreoids. 



Maurer ('83) in Triton, on the other hand, found cells in this 

 region persisting for a considerable length of time, which he 

 interpreted as accessory thyreoid. These he thought were origi- 

 nally part of the isthmus, which eventually formed a group of 

 four or six cells containing colloid, lying in the mid-line between 

 the gen ohyoideus muscle of either side. He says, p. 361, 



In Folge dieser raschen Lageveranderung der Schilddriise, Kurz 

 nachdem sich die oft uniegel massige Theilung vollzogen hat, bleiben 

 haufig, besonders weiter vorn in der Medianlinie zwischen den Musculis 

 geniohyoideis, Theile des urspriinglichen Isthmus liegen, welche spater 

 als vordere unpaare aus vier bis sechs Acinis beste hende colloidhaltieg 

 Nebenschilddriisen sich erhalten. 



In Amblystoma, I am unable at any stage to locate cells in this 

 region which may be interpreted as an accessory thyreoid. There 

 are, to be sure, a few scattered cells, lying in the space between the 

 geniohyoideus muscles and the copula in this region, but these are 

 similar in appearance to the numerous mesenchymatous cells of 

 his same region^ and are not sharply differentiated from them. 

 If these s attered ceils can be looked upon as the remains of the 

 isthmus of the two halves, it is clear that they are rapidly degen- 

 erating, and have lost the staining quality which is still retained 



