THE TRANSVERSE PROCESSES OF NECTURUS 555 



latter before there is the shghtest connection between the para- 

 pophysis and the rib-bearer. Three points are thereby made 

 clear : 



1. The rib-bearer is not an upgrowth of the parapophysis. 



2. The distal end of the parapophysis (basal stump of Goep- 

 pert) does lose its attachment with the rib and does not become 

 attached to the rib-bearer until after the rib has connected with 

 the same structure. 



3. The rib simply shifts its attachment from the basal stump 

 to the rib-bearer as the horizontal septum moves upward. 



After the attachment of the rib to the rib-bearer the para- 

 pophysis may develop dorsolaterally and fuse with the rib-bearer 

 secondarily. This connection may again be lost after ossification 

 sets in. This will be discussed more fully later. 



Another possibility is that the parapophysis may be suppressed 

 altogether. This is seen to be the case in the third vertebra of 

 a 25-mm. larva (fig. 21). Here haemapophyses do not appear 

 at all and the presence of one or two cartilage cells next to the 

 notochordal sheath is all that marks the position of the para- 

 pophysis. The ventral end of the rib-bearer projects downward 

 and inward toward the notochord, terminating in a weak strand 

 of cells which extends mesally to the vestige of the parapophysis. 

 The rib is directly continuous with the ventral end of the rib- 

 bearer (fig. 21, r.). 



If this vertebra be compared with that of a similar stage in 

 Amblystoma (fig. 22), it will be seen to be strikingly similar. The 

 chief difference is that the rib-bearer of the latter is continuous 

 with the cartilage of the neural arch. However, many instances 

 of a direct connection between these two elements were found in 

 Necturus, although they are usually separated. (Compare rib- 

 bearer of opposite sides in fig. 19.) Furthermore, in place of the 

 weak strand of cells between the rib-bearer and notochord found 

 in Necturus, in Amblystoma there is a thin bar of bone. 



