:U*2 DEVELOPMENT OF THE CARAPACE OF THE CHELONIA. 



Iu the Chelonia there are no muscle plates, and the perichondrium is absent from the 

 rib cartilage, except in such situations where ribs are actually seen, as at the outer part 

 of the carapace of turtles. 



In those Chelonia where the carapace forms a complete bony box, the body wall 

 segments into rib cartilages embedded in embryonic connective tissue. Ossification 

 commences on the surface of the rib as in the typical vertebrate, but as there is no 

 periosteum, it spreads outwards and involves the whole body- wall, the tissue of which 

 becomes osteogenetic at first near the rib cartilage, and finally throughout its entirety. 



In those Chelonia in which the carapace is incomplete, the rib expansions involving 

 only a part of the body-wall, the following changes occur. The body-wall segments as 

 before into rib cartilages without true periosteal sheaths, embedded in embryonic con- 

 nective tissue. Bone forms on the outer surface of the ribs and expands more or less 

 into the surrounding connective tissue. This does not occur with great rapidity how- 

 ever, and the intercostal embryonic connective tissue differentiates into white fibrous 

 tissue. 



The neural plates are developed in a manner which is in every way essentially similar 

 to the development of the costal plates. 



DESCRIPTION OF PLATE. 



Figs. 1, 2, 3, and 4. — Transverse sections through the ribs and costal plates in developing tortoise. A, rib- 

 cartilage ; B, scutes ; C, young bone ; D, suture between approaching costal plates. 

 Figs. 5 and 6. — Transverse sections through the carapace of developing tortoise at the costo-marginal junction. 



A, marginal plate ; B, scutes ; C, costal plate. 



Figs. 7, 8, 9, and 10. — Transverse sections through the vertebrge of developing tortoise. A, costal cartilage; 



B, vertebral cartilage ; C, costal bone ; D, vertebral bone ; M is placed on the dorsal muscle. 



Fig. 11. — Part of a transverse section through a rib of an embryo turtle. A, rib cartilage ; B, a few laminae of 

 bone ; C, a lateral projection of bone ; D, osteogenetic tissue around rib, there being no fully-formed 

 periosteum ; H is placed in a blood capillary ; K, osteoblast. 



