progressive transformations of the hyobranchial theory of phyloembryogenes i s. Now it can be said 

 apparatus. The high evolutionary stability of that not only the characteristics or traits of 

 this apparatus is explained by the fact that its definitive organization but also of embryonic 

 action permits the animal to capture prey at a adaptation are subject to the ordered manner of 

 distance; it thus represents the perfection of development which was determined by A. N. Severtov 

 the basic function of feeding. Both reduction 

 and progressive transformation of the hyobran- 

 chial apparatus are most noticeable in the 

 Proteidae, in which only two pairs of gill 

 openings are preserved. Theoretically reduc- 

 tion might be expected to have proceeded fur- 

 ther in the urodelan larvae, since only the 

 single anterior pair of gill openings is 

 necessary for expulsion of water from the 

 oro-pharyngeal cavity. However, the hyo- 

 branchial apparatus cannot be reduced further. 

 The complete loss of gill openings, observed in 

 some adult Amphibia, would if carried out in all 

 stages mean a basic transformation of the mechan- 

 ism of hyoid action. Thus the tendency for a re- 

 duction of the hyobranchial apparatus in the 

 evolution of urodelan larvae is offset by the 

 need to perfect its action in other functions. 

 The hyobranchial apparatus in the larvae of 

 Anura arose by modification of the hyobranchial 

 apparatus in the larvae of stegocephal ids and 

 was formed according to the same principle as 

 in the larvae of extant Urodela. The appeal — 

 ance of the breathing function in the hyo- 

 branchial apparatus of the tadpoles seems to 

 have had no effect on its evolution. The 

 transformation of this system took place as a 

 result of a change in the correlated connec- 

 tions of the hyobranchial apparatus and the 

 organs surrounding it in the ontogenesis of 

 anuran larvae. The deviation which is defined 

 or determined by a change in the area of oscil- 

 lation for the tadpole hyobranchial apparatus 

 is a most important factor which determined its 

 further evolution. It seems to have led to 

 the perfection of a new mechanism for action, 

 that is to a proliferation of cartilage and a 

 corresponding reconstruction of the musculature. 

 The most extreme expression of this tendency 

 among the forms is the construction of the hyo- 

 branchial apparatus in the tadpoles of the 

 species Xenopus laev i s . In all of our work, the 

 data obtained on the ontogenesis and the morpho- 

 logy of the hyobranchial apparatus in amphibian 

 larvae and the comparison with the structure of 

 the hyobranchial apparatus of stegocephal ids 

 and crossopteryg ian fishes confirm the theory of 

 monophyletic origin of the Tetrapoda. 



In our investigation of the evolution of 

 the hyobranchial apparatus in amphibian larvae 

 we examined the evolution of the caenogenetic 

 system, i.e., the complex provisional or tempor- 

 ary adaptations which serve only in the course of 

 the larval period of life. At the time of meta- 

 morphosis, the hyobranchial apparatus becomes 

 subject to the basic morpho-f unct ional recon- 

 struction. The mechanism f or the evolutionary 

 transformation of temporary traits has not 

 previously been investigated. The proofs 

 gathered from our work that embryonic adapta- 

 tion evolved by means of phyloembryogenes is. 

 They also let us extend the boundaries of the 



