310 EMBBYOLOGY OF THE LOWEE VEETEBEATES oh. 



Posteriorly the occipital plates have spread forwards but are still 

 separated by a distinct space from the true chondrocranium. The 

 precociously developed occipital arches (oca) have reached a large 

 size and the pair of corresponding ribs — the occipital ribs (per) — 

 which are so characteristic a feature of the Dipnoan skull have also 

 made their appearance. 



In the next stage figured (Fig. 154, C) the occipital plates have 

 become continuous with the parachordal cartilages forming a broad 

 basilar plate (b.p) in which is embedded the notochord, except its 

 tip which is still to be seen projecting freely into the basicranial 

 fontanelle but which later on disappears. The side wall is extending 

 dorsalwards and has enclosed the roots of the trigeminal and facial 

 nerves, forming the outer wall of the Gasserian recess. Further 

 forwards the front part of the basicranial fontanelle has become in 

 great part obliterated by cartilage continuous laterally with the 

 anterior extensions of the trabeculae. The floor of the cranium is 

 still deficient except anteriorly and posteriorly. Laterally a long 

 antorbital process (ao.p) has grown out from the dorsal edge of 

 the trabecula, passing forwards into the upper lip. 



The olfactory organ has by this stage become enclosed in a char- 

 acteristic olfactory capsule. From the anterior end of the internasal 

 septum a horn-like outgrowth spreads outwards and backwards to 

 form the ventral edge of the capsule, meeting posteriorly an independ- 

 ently developed subnasal cartilage (Fig. 155, C, s.n.c). This horn-like 

 cartilage is met by four cartilaginous outgrowths from the dorsal side 

 of the internasal septum which arch forwards and outwards over the 

 olfactory organ. The roof of the olfactory capsule owing to this mode 

 of origin has a characteristic fenestrated appearance. Between and in 

 front of the olfactory capsules the internasal septum comes to project 

 forwards slightly as the prenasal cartilage (Fig. 154, C, pn.c). 1 



In the last stage figured (Fig. 155, C) the chondrocranium has 

 reached practically the condition of the adult. The occipital arches 

 have extended dorsally so as to fuse, on the one hand, with one 

 another to form the median supraoccipital ridge, and, on the other, 

 with the auditory capsule. A horizontal shelf of cartilage has 

 grown outwards from the side wall of the cranium and quadrate 

 cartilage (see below) enclosing a space (P.O) in which lies Pinkus's 

 organ (see p. 133). The dorsal portion of the internasal septum 

 extends backwards slightly as the mesethmoid cartilage (me). 



1 When investigating the development of Le'pidosiren in South America in 1896 I 

 was struck by the fact that badly macerated skeletons of about this stage, with the 

 lower jaw and other cartilaginous arches detached, as is commonly the case, and with 

 the olfactory capsule frayed out at its edge, presented a remarkable resemblance to 

 the remains of the curious little "lamprey" Palaeospondylus described by Traquair 

 (1893). The resemblance was such as to leave little doubt in my mind that Palaeo- 

 spondylus is really a Dipnoan — either larval or an adult form of small size and 

 primitive structure. This conclusion is supported independently by the investiga- 

 tions of W. and I. Sollas (1903) who conclude that Palaeospondylus is an Amphibian. 

 Any one without practical knowledge of young Lung-fishes Would quite naturally 

 suppose their imperfect remains to be those of young Amphibians. 



