Of THE TONGUE OE PAEEOTS. 239 



Stage 2. 



In Tyrrhiira leucotis (PL XXVII. fig. 19, tli.g.) the muscle is similar to that in Conurus 

 cactorum ; but there is an incipient plane of division existing between that part of the 

 muscle arising from the parahyal process (= thyrohyoideus) and that part arising from 

 the tendon of the hypoglossus obliquus immediately behind the posterior extremity 

 of the entoglossum (= rudimentary thyroglossus) In Cyanolyseus imtagonicus and 

 Conurus jendaya (PI. XXVI. fig. 9) this incipient division extends backwards, in 

 the latter Parrot almost to the posterior end of the muscle, so that there is now 

 formed a definite and independent thyroglossus, having a separate origin of its own, 

 but merged at its insertion with the posterior part of the thyrohyoideus. The extent 

 of the division in Cyanolyseus is intermediate between that in Conurus jendaya and 

 C. cactorum. 



Thus this method is characterized by the fact that the parent muscle (thyro- 

 hyoideus) first grows forwards beyond its origin on the parahyal process and forms a 

 second one at the entoglossum [Brotogerys and C. cactorum) ; then an incipient plane 

 of division arises anteriorly, which lies between the part of the muscle arising from 

 one origin and that from another {Pyrrhura) ; and, finally, this plane of division 

 becomes more perfect, almost dividing the parent muscle into two independent parts 

 [Conurus jendaya and Cyanolyseus). The next stage would be that in which the 

 division is complete and would result in the formation of two independent muscles, 

 producing a condition concerning which it would not be possible to say whether it had 

 arisen by one method or the other. Such a condition we will now consider. 



The Convergence of Methods A, B, C. 



Stage 3. 



The result of the evolution of the thyroglossus by either Method A or B is to 

 produce a muscle Avhich has wholly or in part lost its origin, and whose free extremity 

 lies in or near the middle line in front of the thyroid ; Chrysotis viridigena {ante, 

 p. 236) in Method A foreshadows the further evolution of the muscle, and indicates 

 that the entoglossum is destined to constitute the new origin. In Method B there is 

 nothing to definitely indicate the future origin of the muscle which is withdrawing from 

 the old one, but the a priori probability is that it will be the same as in Methods A 

 and C. In Method C the entoglossal origin is reached at once before the definite 

 formation of the muscle that is to arise from it has taken place. In all three Methods 

 two stages in the evolution of the muscle may be discerned, and the third stage (that 

 now to be described) is common to all. 



The characters that distinguish this stage are: the entoglossal origin and the 

 complete separation of the thyroglossus from the parent muscle (thyrohyoideus). The 

 entoglossal origin is common to this stage and the two stages in Method C, but tliey 



