or THE TONGUE OE PAEEOTS. 235 



Thyroglossus Muscle. 



The condition which this muscle exhibits is interesting. Its evolution starts from 

 three distinct initial stages, all of which meet at a higher stage by convergence, and 

 thence pass to the still higher ones by a common road. The evidence shows that 

 the thyroglossus has arisen from the thyrohyoideus, and that it has done so by three 

 methods: (A) the thyroglossus arises as a branch which separates from the thyro- 

 hyoideus as soon as the plane of division is formed, the latter commencing a,t the 

 anterior end and gradually extending posteriorly ; (B) the plane of division is formed 

 in the parent muscle (thyrohyoideus), but the thyroglossus does not immediately 

 separate from it ; (C) the thyroglossus arises as a forward extension of a portion of 

 the thyrohyoideus, which grows forwards to form an origin at the entoglossum without 

 first separating from the parent muscle. 



Method A. 



Stage 1. 



Cacatua roseicajnUa and Cacatua leadheateri exhibit the first stage by this method. 

 In these birds the muscle is represented by a small branch separating from the anterior 

 portion of the thyrohyoideus ; the branch has lost its origin at the parahyal process 

 and its free extremity does not extend beyond the level of the anterior extremity of the 

 thyroid ; the branch is slightly larger in C. leadheateri than in C. roseica])illa. 



Stage 2. 



In Cacatua galerita (PL XXVII. fig. 13, tli.g.), Calopsittacus, Microglossus, Chrysotis 

 ochr'ocephala (PL XXVII. fig. 12, th.g.), Chrysotis cestiva, Eclectus liectoralis (text-fig. 7, 

 p. 231), and Nestor notabilis (PL XXVII. fig. 22, th.g.) the muscle has separated along its 

 whole length from the thyrohyoideus, though still in contact with it, while its free 

 extremity has grown forwards and towards the middle line, and lies in the entoglossal 

 space, well in front of the apex of the thyroid. This stage is thus in advance of the 

 first, in that the muscle has quite separated from the parent one, and in its growth 

 forwards anterior to the apex of the thyroid. Chrysotis panamensis belongs to this 

 stage, but the muscle (PL XXVII. fig. 14, th.g.) at its free extremity is connected with 

 a twig of the sternohyoideus and is confluent with the extremity of its fellow of the other 

 side. This latter condition, or one in which the free extremities of the muscle of either 

 side are in contact, is present in all the Parrots belonging to this stage. But the 

 connection with the sternohyoideus is peculiar, and it is to be noted that the latter 

 is retrogressing — only its fascia remains — at its junction with the thyroglossus. In the 

 absence of a better explanation, we may regard it as an illustration of the tendency to 

 retain old associations, and may assume that in the primitive condition of the muscle, 



VOL. XVI. — PAET V. No. 4. — October, 1902. 2 m 



