THE TRANSVERSE PROCESSES OF NECTURUS 



561 



SUMMARY 



1. The protons of the rib, rib-bearer, and parapophysis are 

 all condensations in the muscle septa, and are therefore continuous 

 at this stage. The proton of the rib and rib-bearer is made up 

 of a greater number of cells, which is in sharp contrast to the 

 proton of the parapophysis in this respect. The parapophysis is 

 made up of a few cells more or less regularly lined up between the 

 notochord and the lateral muscle mass (fig. 2). 



B 



Ch Ch Q^ 



Fig. 28 Series of diagrams to illustrate the development of the parapophysis 

 and haemapophysis in Necturus. A, trunk region; B, tail-trunk transection 

 region. 



2. The first cartilage to form is that of the basal stump which 

 appears as a knob projecting from the notochordal sheath in a 

 latero ventral direction (fig. 28A, b.s.). Later a laterodorsal 

 outgrowth from this becomes the parapophysis (fig. 28A, p.a.). 



3. The first cartilage of the rib appears dis tally, and this later 

 develops mesally (figs. 29 and 30, A and B). 



4. The first cartilage of the rib-bearer appears next to the 

 neural arch (fig. 29A). Later this develops ventrally and fuses 

 with the distal end of the parapophysis, and also develops dorso- 

 caudally over the outer surface of the neural arch (fig. 29B). 



5. The proximal end of the rib is relatively high in the second 

 and third vertebrae; i.e., it is on a level with the base of the neural 



