THYREOID GLAND OF THE TELEOSTS 721 



ity on the part of the foUicles is most unlikely, and the probability 

 is that they are simply passively pushed or pulled as circum- 

 stances may have it. The forces in development unite to make it 

 possible for the thyreoid gland to spread, and so form a greater 

 amount of functional tissue than could be contained in a compact 

 organ situated in the narrow space between the basihyale and the 

 ventral musculature. 



Little is known of the manner in which the thyreoid gland grows 

 and forms new follicles, and contradictory statements are also 

 found in the literature regarding the primary anlage of the organ. 

 It is scarcely conceivable that a vesicular anlage should exist in 

 all fishes except Ceratodus in which Greil observed a solid one. 

 Before the solid outpushing in Ceratodus separates from the 

 pharyngeal wall it is said to become vesicular, a process exactly 

 the contrary to the usual one. 



Amphibians are believed to have a solid bud-like thyreoid 

 anlage. Maurer states that two days after its evagination 

 the thyreoid is solid in the Anura, and W. Mtiller observed a solid 

 anlage in Rana temporaria and platyrrhinus, in which the first 

 lumen appeared in 25 mm. larvae, after the gland had divided 

 into two halves. In the Urodela Maurer records a solid epithelial 

 bud, Livini finds the same in Salamandrina perspicillata and 

 Muthmann in Triton alpestris. Piatt claims that Maurer's 

 description does not apply in all the Urodela, as is shown by the 

 condition in Necturus. 



The reptiles, biids and mammals are said by the majority of 

 observers to show a vesicular thyreoid anlage, which changes into 

 a compact organ from which follicles later originate. Kolliker, 

 however, observed in the rabbit a thickening in the ventral wall 

 of the pharynx, from which a wart-like solid process was cut off. 

 Born also records the same for the pig. (Both authors quoted from 

 Streckeisen). 



This point is of importance in phylogenetic interpretations 

 since our present views regarding the ancestry of the thyreoid 

 gland are mainly based on a similar evagination, that for the endo- 

 style, found in the Tunicates and Amphioxus. 



