THE SKULL OF OHIMvEKA. 



■side, after issuing from the {interior opening of the ethmoidal 

 canal, enters this space and there breaks up into numerous 

 terminal branches. The two layers of the membrane extend 

 into the supram axillary fold, and apparently end in its ventro- 

 anterior edge. The tubules of the outer buccal group of ampullae 

 open on the external surface oral to the supramaxillary fold; and 

 they and the related sacs lie internal to the membrane that 

 lodges , the other tubules, but, as already stated, fibrous sub- 

 dermal tissues are found in the lips and the naso-labial folds 

 that seem to correspond to this layer of the corium, but they do 

 not form a definite membrane. 



In my work on Mustelus (Allis, 1901) no attention was given 

 to the relations of the ampulla? to this fibrous membrane, but 1 

 now find, on re-examining my sections of this fish, that the con- 

 ditions there are strictly similar to those in Chimcera. In the 

 work on Mustelus I came to the conclusion that each ampullary 

 pore of the adult fish indicated, approximately, the place of 

 origin of the related ampullary organ, the long ampullary tubule 

 of certain of these organs being formed by an exceedingly rapid 

 growth of a primarily short tube, that tube being stretched out 

 between the two relatively fixed points represented by the 

 surface pore and the point where the sensory nerve enters the 

 organ. This has been since confirmed by Coggi (1902), and is 

 further confirmed by the conditions that 1 have since found in 

 Chlamydoselachus, the ampulla? of that fish all having short tubes, 

 and the ampullary sacs all lying immediately beneath the related 

 surface-pores. This marked difference in the positions of the 

 ampullary sacs in this fish and those in Mustelus and Chimcera 

 evidently needs explanation, and it would seem as if it must be 

 in some way related to the amount of cranial tlexure at the time 

 the ampullae are developed. When the cranial flexure is at its 

 greatest, those portions of the external surface of the head 

 on which the ampullary pores are found in the adult must lie 

 anterior or ventral to the curved anterior end of the central 

 nervous system, and hence in the region of the future rostrum. 

 If the ampullary sacs and the related nerves were well developed 

 at this time, it would seem as if the tendency would be to 

 hold the sacs there when the cranial flexure was later reduced 

 and the brain drawn relatively backward. The dermal tissues 

 would, on the contrary, probably retain their relative relations 

 to the underlying parts of the brain, and hence also be drawn 

 backward ; and if the ampulla? had already penetrated the fibrous 

 layer of the corium and continued to lie in it, their short tubes 

 would be drawn out into long tubules lying in the fibrous layer 

 of the corium, as is actually the case in Mustelus and Chimora, 

 But if the ampullary sacs were not well developed when the 

 cranial flexure was at its greatest, their tubules could not be 

 stretched, and it would be the related nerve strands that would 

 be lengthened, as in Chlamydoselachus. This would not, however, 

 explain why these organs penetrate the fibrous layer of the 

 corium without wholly perforating it, nor why these organs alone 



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