28 PB0CBEDINQ8 Or TUB 



addition (due to interstitial postcephalic budding) the chorda must 

 be of a comparatively late stage. 



Both these features, chorda and spinal cord, fit into the sketch 

 I have just outlined, but if we consider the spinal cord as an 

 outgrowth from, and therefore a thing later than, the brain, this 

 seems to go strongly against Dr. Gaskell's theory, and this would 

 not be reconcilable with my early Vertebrate. But Gregenbaur's 

 explanation, development from the supra-ojsophageal ganglia of the 

 Invertebrates, is one of those captivating notions which is really 

 nothing but a working hypothesis to account for the dorsal 

 position of the spinal cord. And yet this hypothesis, absolutely 

 wrong in detail, led and became wrapped up in the much more 

 important principle of the foundation of a trunk by backward 

 interstitial budding. As this became dimly recognised as reason- 

 able, the spinal cord explanation benefited by it, although 

 wrongly. 



A few words about the skeletal material, the cartilage. I 

 remember Gegeubaur saying in his lectures, " AUer Knorpel kommt 

 urspriinglich von Aussen." We are only now beginning fully to 

 understand the meaning of that oracular sentence. The cartilage 

 of the Vertebrata is originally an ectodermal, basal membrane 

 product, which then migrates inwards. It does not arise, as the 

 old master himself had taught, and as everybody teaches, in the 

 immediate vicinity of the chorda, there to form arcuaUa or basal 

 blocks, these to form neural and ventral processes, whence 

 ultimately arise the median fin-supporting rays. The process is 

 just the reverse. First rays, lastly basal blocks, culminating in 

 the formation of an axial skeleton with centra. As an aside, 

 I need scarcely mention that this reversed process considerably 

 assists the derivation of the paired fins from a hypothetical 

 lateral fin. 



Another point : since Gegenbaur has stated it positively, there 

 have been persistent attempts to prove that cartilage appears 

 endogenous in the chorda. Personally I think that this belief 

 rests upon faulty, or misinterpreted observations, but if there 

 should, after all, exist such endogenous chordal cartilage, such an 

 endodermal origin would appear quite irreconcilable with the new 

 doctrine of its ectodermal origin. And yet, if Gaskell's explana- 

 tion of the chorda as an early folded-off portion of his new gut is 

 right, then it becomes quite comprehensible how this new gut-wall 

 may still retain some lingering scleroblastic cells, since, according 

 to Gaskell, this gut is partly made out of ventral ectoderm. 



The early Vertebrate I have just reconstructed approaches the 

 Silurian limbless Ostracoderms. PtericJitht/s may be a belated 

 offshoot, still retaining a pair of Invertebrate limblike appendages. 

 Ostracoderms I hold to be the lowest known Vertebrates, not yet 

 Gnathostomes, whether we call them Hypostomes or Agnatha, or 

 even Cyclostomes in a wider sense. 



It is one of Dr. Gaskell's happiest feats to have shown that 



