DEVELOPMENT OF THE CARAPACE OF THE CHELONIA. 339 



the fibrous carapace, some of them towards the advancing costal plate. The spicules of 

 the two plates have loosely dove-tailed into each other to form a 'primitive suture. 



A Study of Sections Transverse to the Length of the Ribs (Plate, figs. 1-4). 



A section through the distal end of the rib of the green turtle of about 5 centimetres 

 long (fig. 1) shows that the structure is entirely cartilaginous. It is embedded in the 

 general osteogenetic tissue of the carapace, which has a "set " around it. It cannot 

 be said to possess a perichondrium. 



Fig. 2 represents a section through the proximal end of the same rib. Here is 

 found the first commencement of ossification, the mesoblastic cells next the cartilage 

 having become converted into osteoblasts, and having formed a complete ring of bone 

 around the cartilage. At the sides of the rib (see fig. 9), in the region of the intercostal 

 spaces, the process is more active, and tiny spicules are seen already to project into the 

 surrounding tissue. These broaden the rib, and are the first indications of the formation 

 of the costal plates seen from this aspect. 



In the fresh-water tortoise, 2*2, 2*6, and 3 centimetres long (fig. 3), the further 

 development of the costal plates may conveniently be traced. Spicules of bone penetrate 

 in all directions, but especially into the intercostal connective tissue. In 4 and 4 '6 

 centimetres tortoises (fig. 4) the process has so far advanced that neighbouring costal 

 plates have met together, forming the primitive sutures already described, and all the 

 tissue of the carapace except the sensitive layer under the scutes is converted into 

 spongy bone. The rib cartilage is all absorbed, and so is the little tube of bone which 

 first invested it (A, fig. 4). 



The Osteogenetic Tissue of the Carapace. 



The whole carapace of the embryo tortoise, as has previously been mentioned, 

 consists of embryonic connective tissue, in which are embedded cartilaginous vertebras 

 and ribs, the whole being covered with epidermic scutes. The tissue is rich in blood- 

 vessels, and consists chiefly of interlacing collagenous fibres, many of which are 

 straight and clasped by clasping cells. Many young corpuscles are seen evidently 

 about to form fresh fibres, traces of which are often apparent. There is no sign 

 of any segmentation of this osteogenetic tissue. It forms an unbroken sheet of tissue, 

 in which, of course, the cartilaginous ribs and vertebras are embedded. In fig. 11 

 the tissue is represented surrounding a rib cartilage. The connective tissue cells near 

 the cartilage have become osteoblasts, and a thin layer of bone has been formed on the 

 rib cartilage. At the same time, however, active changes take place in the surround- 

 ing tissue, especially in that part which is intercostal. The tissue becomes very 

 vascular near the rib cartilage, and there is a rapid formation of the straight fibres or 

 spicules destined to be included in the forming bone as Sharpey's fibres. In the inter- 



