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 in 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. 
Norr. — 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. 
