92 



THE ANCESTRY OF VERTEBRATES 



showed in different Te\eosteans(Gobius, Hippocampus, Belone), 

 'that the oral aperture at first opens at both its lateral 

 extremities, remaining closed in the middle for some time. 

 Afterwards the two openings fuse. The same was found 

 in Batrachus tau by Miss Platt (1891), and by BOEKE (1904) 

 in Muraenoids. In other Teleosteans. however, the mouth 

 again originates as a single opening from the beginning, 

 . as is the rule in other Craniates. This however does not 



Fig. 18. Head of an embryo of Torpedo ocellata 

 in stage I-K. 



c. heart, hy. hyoidarch, m. mandibular 

 arch, o. anditory vesicle, p. pericard, sp. 

 spiracle, urs. myotomes, 



after Ziegler 1908, p. 657- 



"preclude the possibility that phylogenetically the mouth may 

 liiave arisen from the fusion of two gill-clefts. In young 

 -embryos of Elasmobranchs, as shown by fig. 18 (after 

 .Ziegler, 1908), the spiracle and mouth form the natural 

 ^forward continuation of the series of gill-slits. The same 

 holds for Amphibians. Miss Platt (1896ft. p. 560) writes: 

 "The development of the mouth in Necturus seems to me 

 -4o furnish as striking an argument in favour of its branchial 

 'Origin as the paired oral invaginations found in the Teleostei. 

 The hyobranchial clefts are primarily inclined to the trans- 

 verse plane as above described, the hyomandibular are 

 -Still more inclined, and the oral clefts appear to be abso- 

 lutely fused in a horizontal plane in consequence of the 

 >horizontal position in which the mandibular arches first 

 tlie. Such a supposition readily explains the shape of the 



