DEVELOPMENT OF THE CARAPACE OF THE CHELONIA. 337 



joined posteriorly over the neural canal. The rib is joined by fibro-cartilage both to the 

 notochordal cartilage and to the lateral plate of its side. No trace of bone is as yet 

 apparent. 



The further stages of development I have studied both in the green turtle and in the 

 common fresh-water tortoise. As my specimens of the latter form a more complete series 

 they will now be referred to, but if in any detail they differ from the green turtles, that 

 difference will be indicated. 



A tortoise of 2*2 centimetres long (fig. 9) has the vertebrae composed of one piece of 

 cartilage, complete fusion of the original pieces having taken place. The rib is now com- 

 pletely joined to the vertebral column, but the line of juncture is indicated in this and in 

 older specimens by a curved band of closely-packed cells. I think there can be no doubt 

 that what has been termed the costal cartilage is indeed a true rib cartilage and not a 

 developed transverse process as Gegenbaur affirms. The rib cartilage is from first to 

 last quite distinct from the vertebral cartilages, whereas a transverse process arises as a 

 prolongation of one of these. In the 2*2 centimetres tortoise ossification has already 

 commenced both on the vertebral and on the rib cartilage. The shaft of the rib cartilage is 

 covered by a thin layer of bone, which extends over part of the head of the rib cartilage. 

 This layer of bone, neither in this nor in any of the tortoises of larger size, never extends 

 quite up to, or blends with, the bone which is now found to encrust the vertebral cartilage. 

 In addition to the crust of bone on the rib cartilages, bony spicules project dorsally over 

 the muscle plate (C, fig. 9), and this growth is one of the first indications of the forma- 

 tion of the costal plates. In the same specimen! bone has already formed within the 

 neural canal, and the neural arch is covered posteriorly by a thin crust of bone which 

 extends laterally towards, but never blends with,, the crust of bone on the rib cartilage. 

 From the posterior part of the neural arch bony spicules are seen to have extended into 

 the connective tissue found lying between the vertebrse and the superficial scutes, to form 

 the neural plates. This connective tissue consists partly of those straight interlacing 

 fibres (in connection with which are numerous connective tissue corpuscles) which are so 

 characteristic of a membrane destined to be converted into bone. 



In a tortoise 2 6 and in another 3 centimetres long, the spongy bone developed 

 behind the ribs and vertebrae had increased in amount, extending round and partly 

 enclosing the muscle plate. 



In a specimen 4 centimetres long (fig. 10), the vertebrae are seen to have been covered 

 and lined by bone,, which has extended backwards to form the neural plate. Much of the 

 cartilage of the vertebrae has been removed and replaced by bone, and the same obtains 

 in the rib cartilage. Of the latter the head alone remains intact, together with some 

 scattered and often isolated cartilaginous remains of the main body of the primitive rib. 



The formation of the neural plate is now far advanced. It extends nearly up to the 

 tortoise-shell, and laterally it has extended to meet the advancing costal plate. At this 

 point the bony spicules of the two plates dove-tail very loosely into each other, forming 

 what may be termed a primitive suture. The spicules of the neighbouring plates 



