CHONDKOCKANIUM 



313 



though in all probability variation occurs as between different genera 

 and species and possibly even between individuals of the same species. 

 The trabeculae are strikingly different in their relations during 

 early stages from those of the Dipnoan. Instead of being continuous 

 with the parachordals they are at first separated from them by a 

 wide gap in which appears on each side a small nodule of cartilage 

 the Polar cartilage (van Wijhe, 1905). Further the long axes of 

 trabeculae and parachordals instead of being in line are practically 

 at right angles to one another. It is probable that both of these 

 peculiarities are to be associated with the greatly developed cerebral 

 flexure. The fore-brain has as already described been bent down- 

 wards into a kind of retort shape, the floor of the thalamencephalon 

 coming to face in a tailward direction. As the thalamencephalic floor 

 has undergone this 

 displacement the tra- 

 beculae have been 

 carried with it, so as 

 to assume a practi- 

 cally dorsiventral 

 direction, and this 

 same movement has 

 probably brought 

 about the severance 

 of their original con- 

 tinuity with the para- 

 chordals. It will be 

 noticed from Fig. 156 

 that the displacement 

 has gone even further 

 than has been indi- 

 cated so far, for the 



trabecula has been translated bodily in a tailward direction so that 

 it lies in a plane considerably posterior to the level of the anterior 

 end of the parachordals. 



At the front end of the parachordal a plate of cartilage (Jug. 

 156, o.t) develops in the side wall of the cranial cavity correspond- 

 ing 'generally with the orbito-temporal plate of the Lung-fish (Ali- 

 sphenoid plate, Sewertzoff ; Sphenolateral, Gaupp). This is described 

 by van "Wijhe as being an outgrowth from the parachordal, while 

 Sewertzoff states that it is at first distinct. > 



The auditory capsule originates according to Sewertzoff as a 

 simple outgrowth from the parachordal which gradually spreads 

 outwards and dorsalwards round the otocyst, while according to van 

 Wijhe the first trace of cartilage is on the outer side of the otocyst 

 and is independent. 



The cartilage belonging to the various elements which have been 

 mentioned spreads outwards from each till they form a continuous 

 trough -like chondrocranium. The trabeculae become continuous 



Fig. 156. — Chondrocranium and visceral arch skeleton of 

 an embryo of Acanthias. (After Sewertzoff, 1899.) 



a.e, auditory capsule; &, £ 5 , branchial arches; H, hyoid ; M, 

 mandibular arch ; o.t, orbito-temporal ; p.e, parachordal ; tr, trabecula. 



