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Dr. F. M. Balfour and Mr. W. N. Parker. [Dec. 8 y 



of contiguous vertebrae, the curved line where this will be effected 

 being plainly marked out at a very early stage. As these rings are 

 formed originally by the spreading of the cartilage from the primitive 

 neural and haemal processes, the intervertebral cartilages are clearly 

 derived from the neural and haemal arches. The intervertebral carti- 

 lages are thicker in the middle line than at their two ends. 



In the latest stage examined (11 centims. long) the vertebral con- 

 strictions of the notochord are rendered much less conspicuous by the 

 intervertebral cartilages giving rise to marked intervertebral con- 

 strictions. In the intervertebral regions the membrana elastica 

 externa has become aborted at the posterior border of each vertebra, 

 and the remaining part is considerably puckered transversely. The 

 inner sheath of the notochord is puckered longitudinally in the inter- 

 vertebral regions. The granular external layer of the sheath in the 

 vertebral regions is less thick than in the last stage, and exhibits a 

 faint radial striation. 



Two closely approximated cartilaginous elements now form a key- 

 stone to each neural arch above ; these are directly differentiated from 

 the ligamentum longitudinale superius, into which they merge above. 

 An osseous plate is formed on the outer side of each of these carti- 

 lages. These plates are continuous with the lateral osseous bars of the 

 neural arches, and give rise to the osseous part of the roof of the 

 spinal canal of the adult. Thus the greater part of the neural arches 

 is formed by membrane bone. 



The haemal arches are invested by a thick layer of bone, and 

 there is also a continuous osseous investment round the vertebral 

 portions of the notochord. The intervertebral cartilages become pene- 

 trated by branched processes of bone. 



The embryological part of this section is followed by a comparative 

 part treated under three headings. In the first of these the vertebral 

 column of Lepidosteus is compared with that of other forms ; and it is 

 pointed out that there are grave difficulties in the way of comparing 

 the vertebrae of Lepidosteus with those of the Urodela, in the fact 

 that in Lepidosteus the intervertebral cartilages originate from the 

 bases of the arches, while in the Urodela they are stated by Gotte 

 to be thickenings of a special cartilaginous investment of the noto- 

 chord, which would seem to be homologous with that cartilaginous 

 sheath which is placed in Elasmobranchii and Dipnoi within the 

 membrana elastica externa. On the other hand, the development of 

 the vertebrae of Lepidosteus is shown to resemble in most features that 

 of Teieostei, from which it mainly differs in the presence of interver- 

 tebral cartilaginous rings. 



In the second section, devoted to the homologies of the ribs of 

 Pisces, the conclusions arrived at are summed up as follows : — 



The results of the authors' researches appear to leave two alterna- 



