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XV. — The Development of the Carapace of the Clielonia. By John Berry Haycraft, 



M.D., D.Sc, F.R.S.E. (With Plate.) 



(Read 28th February 1890.) 



All observers are agreed that the bones of the plastron and some bones of the cara- 

 pace are simple membranous bones, arising from centres formed in a pre-existing fibrous 

 membrane. Considerable difference of opinion exists as to the development of the 

 costal and neural plates of the carapace. 



Owen # speaks of the ossification " as extending from the ribs and neural spines into 

 the substance of the neural and costal plates. The ribs and spines enter into the com- 

 position of the carapace." 



GEGENBAURt questions whether the ribs of the Chelonia " are not in reality enormously 

 developed transverse processes, and considers that the neural and costal plates have 

 developed in the integument." 



ClausJ remarks that the spinous processes of eight of " the thoracic vertebrae (2nd to 

 9th) appear in the middle line as horizontal plates (neural plates), the ribs of the same 

 vertebrae are transformed into broad transverse plates (costal plates)." 



Huxley § says that the neural plates and the costal plates exist as expansions of the 

 cartilages of the neural spines and ribs of the primitive vertebras, before ossification 

 takes place. This being the case, the " neural and costal are vertebral and not dermal 

 elements, however similar they may be to the nucleal, pygal, and marginal plates." 



It would be difficult to find an example of greater confliction of opinion than is 

 contained in the above statements, and it is obvious that even the simple facts 

 of the developmental process have not been made out. The most recent contributions 

 to the subject are those of Dr C. K. Hoffmann. f| This observer has carefully 

 described the main features of the development both of the costal and neural plates. 

 He has shown that the primitive vertebras and ribs, composed of cartilaginous 

 tissue, become encrusted with bone, but he erroneously describes the costal and neural 

 plates as arising outside the vertebral and costal periosteum. In reality, as I shall show, 

 there is no true periosteum at all. His view, undoubtedly incorrect, is that these plates 

 arise outside the ribs and vertebras, and are not expansions of them ; they are mem- 

 branous ossifications immediately surrounding the ribs and vertebras. He describes very 

 fully the subsequent calcification and absorption of the costal and vertebral cartilages, 

 and the obliteration of the primitive bony rib. 



* Gomp. Anat. and Phys. of Vertebrates, vol. i. p. 63. 



+ Elements of Comparative Anatomy, pp. 433, 440. 



% Text-Book of Zoology, p. 226. 



§ Tlxe Anatomy of Vertebrate Animals, p. 201. 



|| Dr H. G. Bronn's Klassen v. Ordnungen des Thier-Reichs., Sechster Band, iii. Abtheilung. 



VOL. XXXVI. PART II. (NO. 15). 3 G 



