340 DR JOHN BERRY HAY CRAFT ON THE 



costal region this change is most evident, and by the time some half dozen laminae of 

 bone have been deposited on the rib cartilage, well-marked processes of spongy bone are 

 seen to have already projected into the intercostal spaces. 



The processes are furnished at these points by little brushes of straight connective 

 tissue fibres, between which osteoblasts are found in great numbers. Parts of these 

 straight fibres are already lodged within the processes from which they project, and the 

 further growth of these processes is, of course, due to the further inclusion of these 

 and of other fibres. In this way, then, the whole of the membranous carapace becomes 

 converted into spongy bone, which grows, and is subsequently divided into two, the 

 outer and the inner, compact plates. These changes are brought about by that modelling 

 process, seen in all osseous structures, and which seems to consist in a constant deposi- 

 tion of fresh bone and removal by absorption of superfluous tissue. 



What is a Costal Plate ? 



Let us first consider the development of the costal plate, for the neural plate, 

 developed in precisely the same way, may be included in any deductions we may draw 

 concerning the homologies of the rib plate. 



In the first place, are these intra-cartilaginous or intra-membranous bones ? Hoff- 

 mann and others maintain that they are intra-membranous like the other bony scutes, 

 because they are developed in membrane, and because the rib cartilage around which 

 this development takes place is rapidly absorbed. The fact that these plates have been 

 developed in membranes around the cartilages of the rib, does not, however, make them 

 membranous bones ; unless, indeed, one looks upon the femur or a human rib as mem- 

 branous bones. I suppose that it would be safe to say that not one single ounce of bone 

 in an adult human skeleton was ever in histeogenetic connection with cartilage at all. 

 The latter tissue is, as far as long and short bones are concerned, a primitive tissue 

 indicating their future positions, but in no way developing into them. The shaft of the 

 femur and of a human rib are developed in the membranes which surround the primitive 

 cartilage, which membrane is called the periosteum. The only way we can possibly look 

 at the question in the light of modern histology is not whether or not bone is formed 

 out of membrane, but whether or not its position was taken at an early period of 

 development by hyaline cartilage. From this point of view the answer is simple. The 

 costal plate differs entirely from a plastral plate, in that the latter was never in any way 

 connected with cartilage, while the rib pla.te was, for it developed like the human or any 

 other rib around a rib cartilage. It is true that the rib plate developes around the rib 

 cartilage in a different manner from that in which the human rib developes, but this does 

 not alter the morphological homologies of these structures. The supposed hard and fast 

 lines in animal morphology do not exist in fact, and cell structures and cell processes 

 pass one into another through innumerable transition forms. At one time one or two 

 typical osseous developments had been described, but it is now known that these types 



