:l:N DR JOHN BERRY HAYCRAFT ON THE 



never, however, come actually into contact one with another. They are invariably 

 ensheathed and separated from one another by osteogenetic fibrous tissue loaded with 

 osteoblasts. As the plates grow in size, the suture still remains ; there is no bridging 

 over of spicules from one plate to another. 



Larger tortoises simply show further stages of growth. The costal and neural plates 

 grow in size, and the vertebral cartilage entirely disappears and is replaced by bone. 

 The fibro-cartilaginous carapace is now replaced by bone, which, forming on the surface 

 of the cartilage, gradually extends in all directions involving the cartilage itself, and the 

 whole of the fibrous carapace except a thin layer under the chitinous carapace, which is 

 converted into ordinary fibrous tissue. 



The development of the neural plate has now been fully described, except, indeed, 

 the process by which the cartilage and the fibrous tissue change into bone. It will be 

 convenient to study this when the development of the costal plate has been more fully 

 described. The formation of the latter at its proximal extremity has been already 

 studied ; it will be advisable to examine a series of sections transverse to the length of 

 the tortoise, but passing through the rib cartilage and developing costal plate at its 

 distal margin, and showing its relation with the marginal plates. 



Sections at the Costo-Marginal Junction (Plate, figs. 5 and 6). 



It will be sufficient to study two sections, one of the tortoise 2 '6 centimetres and the 

 other of the tortoise 4'6 centimetres long. In the figure (fig. 5, A) the commencing 

 marginal plate is shown as well as the external end of the costal plate. The latter 

 contains the still cartilaginous rib, absorption of this tissue not having been completed. 

 From the outer end of the rib cartilage (which is not represented in the figure) bony 

 spicules (C, fig. 5) shoot out towards the marginal plate. The spicules have not nearly 

 reached the marginal plate, and are still far away from the notch between the superficial 

 scutes of the carapace. 



In the 4 "6 centimetres tortoise, the marginal plate has increased considerably in size. 

 The rib cartilage is seen here and there within the costal plate. Most of the cartilage is 

 indeed absorbed, but portions are still to be seen, and occasionally almost all the rib cartilage, 

 is left. The rib cartilage is much larger, and it is thicker than the rib cartilage of the 2'G 

 centimetres tortoise. It seems, then, that during the formation of the costal plate, even 

 up to a comparatively late period, the cartilage continues to grow, and the absorption 

 of large masses of the tissue does not seem to interfere with the growth of those portions 

 which remain within the bony ring in which they are enclosed. Not only has the rib 

 cartilage grown outwards towards the marginal plate (fig. 6), but the bony spicules have 

 pushed outwards, and have passed beyond the notch on the superficial scutes. The 

 marginal plates, which arise as little nodules in the connective tissue, are seen in the 4 '6 

 centimetres tortoise to have increased considerably in size. Spicules have shot out into 



