556 D. L. GAMBLE 



DEFINITIVE ESTATE 



While the body of the animal is growing the ribs must shift 

 laterally to come into their final position, thus necessitating the 

 elongation of the transverse processes. This is interfered wdth 

 by the formation of bone around these structures. Hard skeletal 

 parts always interfere with the growth of softer parts, and provi- 

 sion must always be made for it. In higher animals bone growth 

 and absorption go hand in hand. In Necturus, however, no 

 resorption of bone deposited around the transverse process and 

 rib was observed. Growth is provided for by the failure of bone 

 to form over the point of union between the proximal end of 

 the rib and the distal end of the parapophysis. Furthermore, 

 between these two elements is found procartilage cells which by 

 proliferation bring about a lengthening of the parapophysis. 

 In principle it is similar to the lengthening of the long bones 

 through the proliferation of cells in the diapophysial plate. This 

 point of growth together with the one at the distal end of the rib 

 makes possible the accommodation of these structures to the 

 growth of the body. 



As before mentioned, the dorsal head of the rib develops as a 

 dorsal outgrowth from the rib itself. In the trunk this cartilag- 

 inous process does not reach the vertebra, connection being 

 made by a strand of connective tissue. In the second, third, and 

 fourth vertebrae the tubercular head of the rib meets a corre- 

 sponding outgrowth from the rib-bearer. Between the two, pro- 

 cartilage is found and no bone develops over the articulation. 

 This again must be interpreted as a provision for growth, for as the 

 parapophysis grows outward the dorsal part of the transverse 

 process must keep pace with it. The transverse processes of the 

 second, third and fourth vertebrae therefore possess both a ventral 

 and a dorsal cartilaginous rod surrounded by a sheath of bone. 

 Between the dorsal and the ventral rod a thin layer of bone 

 develops which Wilder ('03) calls the vertical lamina. 



Goeppert, in describing the transverse process of Necturus, 

 states that the dorsal part does not appear to be so clearly a rod 

 as it does in Salamandra, and that it lacks the distal excavation 

 seen in Salamandra, the tubercular head of the rib not attaching 

 directly to it. 



