28o SCOTT. [Vol. I. 



it is perfectly clear that there is present a pair of hypoblastic 

 diverticula immediately behind the mouth and in advance of 

 the first permanent gill-pouches. (See my former paper (34), 

 Fig. 38, Taf. X.) These diverticula are in every respect like 

 those which form the permanent clefts, with regard to meso- 

 blastic segments, arterial arch, nervous and skeletal elements, 

 differing only in the fact that they do not seem at any time to 

 perforate the skin. This observation has been abundantly con- 

 firmed by Dohrn (13) and Shipley. Now, if Julin's view that 

 the first pcnnaiie7it cleft is the homologue of the spiracle in 

 the Selachians be correct, the development of Petromyson 

 demonstrates the correctness of the hypothesis advocated by 

 Dohrn and Beard (6), that the hyoid arch is composed of two 

 cranial segments. But, from the evidence now at command, I 

 cannot but conclude that the first permanent cleft is supplied, 

 not by the seventh, but by the ninth nerve, and in consequence 

 that the transitory cleft is the representative of the Selachian 

 spiracle. 



8. The Auditory Nerves. — The eighth pair of nerves opposes 

 several difficulties to the following out of its development. 

 Shipley's account is very brief, and is as follows : " A few fibres 

 from the brain enter the recessus labyrinthi of the ear; these 

 arise close to the root of the seventh, and constitute the eighth 

 nerve." These fibres I have not seen, but they may possibly con- 

 stitute the dorsal root of the acusticus. The main portion of 

 the eighth nerve appears to arise from a common root with the 

 seventh, and its ganglion would seem to be continuous with the 

 facial ganglion (Fig. 39, PI. XI) ; and it is not at all improbable 

 that the ventral root seen in Fig. 42, PI. XI, which joins the 

 facial ganglion, is one of the roots of the auditory nerve. I have 

 not detected any fusion of the auditory ganglion with the epiblast 

 of the auditory involution, although it is very probable that such a 

 fusion takes place ; if so, it must occur before the ganglion is 

 separated from that of the seventh nerve. The auditory 

 ganglion becomes separated from the facial, and lies close 

 against the auditory vesicle, and is in more advanced larvae 

 enclosed within the wall of the cartilaginous ear-capsule (Fig. 

 34, PI. X), and from it fibres may be traced into the membran- 

 ous labyrinth. At this stage the auditory nerve arises from a 

 mass of cells high up in the side wall of the hind-brain, the 



