308 EMBRYOLOGY OF THE LOWER VERTEBRATES en. 



the cranial cavity still extends forwards between the eyes and the 

 trabeculae retain their primitive parallel position some distance apart. 

 The skull in actinopterygian Ganoids, Teleosts, Amniotes and certain 

 Elasmobranchs develops after the tropibasic type while in Amphibians, 

 Lung-fishes, Crossopterygians and some Elasmobranchs it retains the 

 platybasic condition. 



As the platybasic type of cranium is admittedly the more primitive 

 we shall deal with it first and will take as our example the cranium 

 of the Lung-fishes Lepidosiren and Protopterus as described by Agar 

 (1906). 



DEVELOPMENT OF CHONDROCRANIUM IN LEPIDOSIREN AND PROTO- 

 PTERUS. The first rudiments of cranium become apparent about 

 stage 31 in the form of a longitudinally situated condensation of 

 mesenchyme the rudiment of the trabecula lying upon each side 

 beneath the thalamencephalon and mesencephalon. At its front 

 end the trabecula rapidly extends dorsal wards to form a vertical 

 plate of prochondral tissue lying against the side wall of the thalam- 

 encephalon, and terminating in front against the optic nerve. 

 The dorsal portion of this plate, just internal to the deep ophthalmic 

 nerve, is the orbito-temporal process (Fig. 155, A, o.fj. From the 

 outer surface of the trabecula, just in front of the main portion of 

 the trigeminal nerve, there projects outwards a horizontal shelf of 

 cartilage (Fig. 155, A, g.r). This is the rudiment of the portion of 

 cranium which contains the ganglia belonging to the Trigeminal 

 and Facial nerves (Gasserian recess, Bridge). The cranial rudiment 

 becomes prolonged backwards, the backward prolongation represent- 

 ing the parachordal cartilage of meroblastic Vertebrates. 



This parachordal rudiment lies on each side of the front portion 

 of the notochord but, unlike what is more usual in other Vertebrates, it 

 is separated from the notochord by a considerable space (Fig. 154, A). 

 The cranial rudiment so far described gradually becomes chondrified. 

 About this time there appears a condensation of mesenchyme round 

 the outer side of the otocyst : this is the outer wall of the auditory 

 capsule (Fig. 154, A, a.c). A little later than the stage mentioned 

 a knob of cartilage begins to develop on each side of the notochord 

 at the level of the septum between metotic myotomes III and IV. 

 This is an enlarged and precociously developed neural arch which, 

 Miinir, as \\ill IM- seen presently, incorporated in the skull, is 

 known as tin- occipital arch. The base of this spreads forwards 

 :i\ni\ tin; dorsolateral smTaer of the notochord to form the occipital 

 plate (Figs. 154 and 155, B, o.p). 



I'.y stage 34 the < -horn In (cranium has reached the condition 

 .shown in Figs, L54and 155, r. The trabeculo-parachordal cartilage 



pivad outwards and bifll become continuous witli the rudiment, 

 of t hr audit" 1 le BO l hat the greater part of tin- Literal jM.rt.idii 



ol !,he definitive. ohondroOranitHH is now laid down iii cartilage. 

 Tin- two trahuculae have extended forwards, converging towards 

 one another and passing in front into an unpaired IIIUHS of cartilage 



