THE TRANSVEESE PROCESSES OF NECTURUS 547 



Following the law of cephalocaudal growth and differentiation, 

 structures nearer the anterior end of the body have differentiated 

 further than those which are located more caudally, so that in 

 passing backward through the trunk region successively younger 

 stages in the formation of ribs and transverse processes are 

 encountered. 



This law applies to all the trunk vertebrae except the first 

 three. Here, as will be discussed later, the development of the 

 various parts of the transverse process and ribs seems to lag 

 behind the trunk vertebrae immediately caudad. This condition 

 may be interpreted as the beginning of a process which in higher 

 forms leads to a reduction of ribs in the cervical region. 



Beginning with the fourth vertebra, the rib-bearer is seen 

 connecting the neural arch and the distal end of the para- 

 pophysis, while farther caudad this connection has not yet been 

 made and the rib-bearer appears as a knob of cartilage attached 

 to the neural arch (fig. 10). Still farther caudad the cartilage 

 of the rib-bearer has not yet made its appearance. 



In the 43-mm. larva studied by Goeppert this cartilage of the 

 rib-bearer next to the neural arch is found, but he concluded that 

 the lack of continuity between it and the basal stump was due 

 to a secondary degeneration of the cartilage. When intermediate 

 stages are studied, however, it is shown very clearly that cartilage 

 between the two may never have been formed. The study of 

 older stages shows that in the greater number of the trunk 

 vertebrae this connection is finally made, but that in a few in 

 which development is arrested the two are separated. In such an 

 event the result is that of the relations seen by Goeppert and 

 interpreted by him as a secondary degeneration. 



Fig, 6 Transection through the second vertebra of a 20-mm. larva, n.a., 

 neural arch; /i.o., haemapophysis; v.a., parapophysis; r.b.p., rib-bearer proton. 



Fig. 7 Transection through the fourth vertebra of a 23-mm. larva, a. a. p., 

 anterior articular processes; ?i.a., neural arch; r.h., rib-bearer; p.a., parapophysis. 



Fig. 8 Transection through a midtrunk vertebra of a 23-mm. larva, n.a., 

 neural arch; ? .b.p., rib-bearer proton; r.p., rib proton; p.a., parapophysis. 



Fig. 9 Transection through the sixth vertebra of a 24-mm. larva, n.a., neural 

 arch; r. 6., rib-bearer; r.p., rib proton; p.a., parapophysis. 



Fig. 10 Transection through a trunk vertebra of a 25-mm. larva, r., rib; 

 n.a., neural arch; r.b., rib-bearer; p.a., parapophysis. 



