REPTILES. 



Professor Berry Haycraft gives the following account of the develop- 

 ment of the dorsal shield : 



If we compare a very early embryo turtle with that of a crocodile, we 

 notice the following difference : In the crocodile, each cartilaginous 

 rib is completely invested by a tubular sheath of young connective tissue, 

 and in the intercostal spaces are distinct muscle plates. In the turtle the 

 cartilaginous ribs are simply embedded in young osteogenetic tissue, 

 which forms the whole of the body wall, extending superficially up to 

 the skin. As development proceeds in the crocodile, the tubular sheath 

 of connective tissue (periosteum) investing each cartilaginous rib, grows 

 in size, and forms bone (the rib) anteriorly, the cartilage being absorbed. 

 Thus we get the adult cylindrical rib, separated from its neighbours by 

 the intercostal muscles, developed from the muscle plates. In the 

 green turtle bone begins to form upon the rib cartilage, the latter sub- 

 sequently being absorbed, but as there is no investing periosteal sheath, 

 this formation of bone spreads out on all sides, right up to the skin 

 superficially, and as far as the neigh- 

 bouring growths laterally, to form the 

 solid bone of the carapace. In the 

 mud turtles, the growth of bone which 

 is extending laterally from each carti- 

 laginous rib, does not meet its neigh- 

 bour, for already the intercostal tissue 

 has partly become differentiated into 

 fibrous tissue, and a fibro-osseous cara- 

 pace results. In the green turtle, the 

 rib cartilage, at both its distal and 

 proximal ends, is invested by true 

 periosteum, which causes in these parts 

 the formation of cylindrical bone. 



What then is a costal plate ? 



It is more than a rib ; it is a rib, 

 which, in its development, has spread 

 into and involved the surrounding inter- 

 costal tissue. 



Is it an intramembranous or intra- 

 cartilaginous bone? We now know 

 that all bones are developed through 

 the agency of membranes, and that the 

 humerus, for example, an intracarti- 

 laginous bone, is eventually formed en- 

 tirely from its membranous periosteum. 

 A membrane bone is therefore not a bone developed from a membrane, 

 for every bone in the body is now known to be so formed, it is a bone 

 whose place was never represented by cartilage. 



If we accept this view of an intramembranous and intracartilaginous 

 bone, a view forced upon us by modern inquiry, then the costal plate is 

 an intracartilaginous bone, and comes out in its proper contrast from the 

 marginal and plastron plates which are not preformed in cartilage. 

 The neural plates may be looked upon as similar in their origin to the 

 costal plates, bone encrusting the cartilaginous vertebrae, and then ex- 



F I G. 191. Carapace of 

 Tortoise. (From Edin- 

 burgh Museum of Science 

 and Art.) 



The dark contours are those of 

 the bony pieces ; the lighter con- 

 tours are those of the scales which 

 have been removed. 



