WAITE : PLEXI OF NECTURUS. 89 



myotomes in this region is able to produce sacral ribs. In the greater 

 number of cases the function of rib production by the myotome is 

 expressed at a constant distance (measured in metameres) from the 

 cranium, resulting in what we call the normal position ; but as some 

 tendency arises for the girdle to be abnormally placed, such tendency 

 is expressed by the appearance of a new -plexus and a new girdle in a new 

 position. However, as we have seen in group B, the plexus in the new 

 position does not have the same relation to the girdle-bearing segment 

 that it did in the old ; on the contrary, it tends to occupy a place inter- 

 mediate between such a position and the old one. The girdle also may 

 possibly show a parallel intermediate position ; but if so, it is more 

 difficult to verify. Such an intermediate position of the plexus may be 

 interpreted either as representing an atavistic tendency, or an incom- 

 plete migration. I am inclined to believe m the latter, and that it 

 indicates a less complete response by the plexus than by the girdle to 

 the influence of the musculature which changes the locus of both girdle 

 and plexus. 



The nerve relations in Necturus show that variations of girdle and 

 plexus are nearly parallel, but that these are in some degree independent, 

 as exhibited by the fact that the strength-centre of the plexus does not 

 have just the same relation to the girdle in the variant that it had in the 

 normal condition. 



February 20, 1897. 



Note. — Since the manuscript of this paper left my hands, two papers 

 bearing upon the question of intercalation have appeared. 



Ridewood ('97, p. 366) considers that the point of sacral rib forma- 

 tion and attachment is determined by some stimulus external to the 

 column, and that the girdle does not migrate from one segment to 

 another during ontogeny, — a conclusion which is along the same line 

 as my own (pp. 85, 87). 



Baur ('97) contends for intercalation as an ontogenetic process, but 

 I find little in addition to what is contained in his earlier paper (Baur, 

 '91), except that he now (p. 42) supports the idea that the pelvis, devel- 

 oping in one segment, may migrate into a neighboring segment, and 

 become "secondarily united with the vertebral column." 



October 1, 1897. 



