466 BUMPUS. [Vor. XII. 
Those who attempt an explanation of this phenomenon on 
the principle of intercalation, excalation, or of pelvic migration 
meet with most provoking difficulties. Why should only one 
lateral half of a vertebra be intercalated, giving rise to a body 
segment which bears a single rib on one side and two ribs on 
the other? Or why should the process of the formation of 
sacral ribs involve portions of two vertebrz rather than one, 
and thus produce an asymmetrical sacrum? 
The difficulty is partly obviated if we admit that the differ- 
entiation of the sacral vertebra is the result of centripetal 
influence exerted by the growing Anlage of the appendage. If 
the first rudiments of the appendages are not laid down exactly 
opposite each other, and there are many reasons, as, for exam- 
ple, the curvature of the embryo, the pressure of neighboring 
eggs, etc., why their primitive positions might vary, an unsym- 
metrical or oblique sacrum would result. 
It is a remarkable fact that of the eight unsymmetrical 
specimens, all but one, No. 62, have the left half of the sacrum 
in advance of the right, z.e., the axis of the sacrum is sinistro- 
dextral. 
I have been quite unable to find any valid explanation for 
this peculiar uniformity, unless it is based upon the curvature 
of the embryo. Dr. Whitman has informed me that the young 
of Necturus, as it lies upon the surface of the egg, is curved, 
but I have not seen the embryos or larve, and I ‘do not know 
whether the curve is lateral or dorsal-ventral, or even if it is 
fairly constant in its trend. An attempt to correlate the posi- 
tion of the sacral axis with the slightly asymmetrical position 
occupied by the paired viscera has proved futile. 
SEcTION V. 
Is the position of the pelvic arch dependent upon the ordinal 
position of some one segment (sacrum) of the vertebral column, 
or is its position determined by, and is the sacrum the resultant 
of, the location of some topographical point ? 
I do not know that an attempt has ever been made to com- 
pare the distribution of the vertebra and the location of the 

