MOTOR NUCLEI IN PHYLOGENY 407 



respiration in urodeles is carried on by means of the same bucco- 

 pharyngeal pump mechanism, and in anuran larvae by but a 

 slightly modified one, so that the act of aquatic breathing dif- 

 fers in these forms from that of pulmonary respiration only in 

 the absence of the third phase, viz: Inspiration (Babak und 

 Ktihnova, 3; Brtiner, I.e.; Gaupp, 24; Schulze, 51). In this 

 connection Gaupp has pointed out that the phases of aquatic 

 respiration are aspiration — inspiration — (expiration) and that in 

 carrying out these movements the bucco-pharynx acts first as 

 a suction pump and then as a force pump just as it does during 

 pulmonary respiration. 



Gaupp was led to the conclusion that, since there occurs no 

 fundamental change in the mechanics of respiration at meta- 

 morphosis, it was to be expected that the morphology of the 

 'respiratory center' would be the same in larval and adult anu- 

 rans. Baglioni (5) has pointed out that this thesis cannot be 

 accepted in its entirety because at metamorphosis, though the 

 respiratory effector mechanism may not be greatly altered, yet 

 a very fundamental physiological change must occur, since at 

 this time air takes the place of water in furnishing the ' adequate* 

 stimulus initiating normal respiratory reflexes. 



In the urodeles in the case of Cryptobranchus and Triton, 

 though physiological changes may have occurred during the 

 evolution of their exclusively pulmonary type of respiration, 

 the visceral motor nuclear pattern of these animals is practically 

 identical with that of the perennibranchiate form Siren. What- 

 ever the 'respiratory center' in amphibians may be anatomically, 

 it would seem that the visceral motor nuclei of the brain stem 

 in these forms as in fishes, must be very intimately concerned 

 in its make-up. Viewed in this light the phylogeny of the vis- 

 ceral motor nuclear pattern in urodeles points to the essential 

 truth of Gaupp's conception that in itself a change from aquatic 

 to pulmonary respiration need involve no change in the mor- 

 phology of the 'respiratory center' (vide infra). 



Thus among urodeles the same visceral motor nuclear ar- 

 rangement that obtains in perennibranchiate forms (continuity 

 of VII-IX-X motor nuclei) is retained also in exclusively air 



THE JOUHNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



