278 
„showed in different Teleosteans (Gobius, Hippocampus, Belone), 
that the oral aperture at first opens at both its lateral 
„extremities, remaining closed in the middle for some wp 
Aîterwards the two openings fuse. The same was fou 
“in Batrachus tau by Miss PLATT (1891), and by BOEKE (1008 
sin Muraenoids. In other Teleosteans, however, the mout 
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 sed p. pericard, sp. 
spiracle, urs. myoto 
after ZIEL ER 1908, p. 657. 
“preclude the possibility that phylogenetically the mouth may 
«have 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 
:ferward continuation of the series of gill-slits. The same 
“holds for Amphibians. Miss PLATT (18966, p. 560) writes: 
“The development of the mouth in Necturus seems to me 
“to 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- 
Jutely fused in a horizontal plane in consequence of the 
“horizontal position in which the mandibular arches first 
sie. Such a supposition readily explains the shape of the 
