516 MISS JOAN B. PROCTER ON THE 



Origin of the costal and neiiral plates. 



Much has been written on the origin of these plates, and many- 

 ingenious theories have been propounded*. 



Most authors agree that the marginals, nuchal, and pygals are 

 of dermal origin, and that the plasti'on is derived from the 

 sternum and gastralia which have become greatly modified, but 

 the question of the neurals and costals is still unsettled. 



The earliest opinion worth citing is that of Cuvier (1799), 

 who considered that the costal plates were formed " par les dilata- 

 tions de huit cotes ou batons osseux qui prennent naissance sur 

 les unions des vertebres, et se terminent a un rebord qui entourent 

 toute la carapace." The idea that the plates are dilatations of the 

 vertebral spinous processes and true ribs is held by many sub- 

 sequent authors — Geoflfroy St. Hilaire(1809), Bojanus (1819-21), 

 Rathke (1848); or, further, that these plates originate in periosteal 

 or diiferentiated tissue — Goette (1899), Haycraft (1899), Newman 

 (1906). Haycraft (1891) foi'mulates a theory that costal plates 

 are rib-expansions formed where there is no diflferentiated 

 periosteal membrane confining the rib. In the case of turtles, 

 where the expansion does not reach the marginal, he states 

 that the distal portions which preserve their rib-like form are 

 unable to expand because " they are invested by a restraining 

 periosteum." The fact that the costal plates in most Chelonians 

 develop gradually towards the marginals, passing through the 

 stage normal to adult turtles, disproves this theory. 



Cams (1827) and Gegenbaur (1889) thought that the costal 

 plates were greatly developed vertebral transverse processes— a 

 view which, of course, is immediately disproved by the fact that 

 their points of origin are distinct and widely separated from the 

 vertebral column. 



O. P. Hay (1901) accepts none of the foregoing theories. He 

 distinguishes thi-ee layers of bone — dermal, fascial, and cartila- 

 ginous. The first is present in the armour of Dermochelys, the 

 most primitive living Ohelonian. . The modern costal and neural 

 plates are formed not by dermal bvit by the fascial bone, 

 which since it is completely united to the cartilage elements, 

 appears to arise directly from the perichondrium, as observed by 

 Goette (1899). 



That these plates are dermal in origin and similar in every way 

 to the nuchal, pygal, and marginal plates was first suggested by 

 Carus (1834), and was followed up by Peters (1838), Owen (1849), 

 Baur (1887), Gadow (1899, 1905, 1909), and Yersluys (1914). 

 Many points brought to light in the present study corroborate 

 this view. For instance, the fact that the true ribs are 

 degenerating throughout their length by absorption within the 

 periosteum whilst the costal plates are in process of actual 

 development, and also the marked difference in the texture of 



* " Hypothesis follows hypothesis ; the theoretical rubbish-heap accumulates ; 

 and truth ever eludes us." — Fabee (transl.). 



