388 MONTGOMERY. [Vol. XIII. 



expressed there. On a transverse section of a lateral chord 

 one finds, both on the dorsal and on the ventral side (ganglion 

 cells are absent on the median and lateral aspects, as stated by 

 Coe, '95a), a right and a left cluster of these cells (Fig. 25). 

 For each radial group is one opening in the inner neurilemma, 

 serving for the transmission of its bundle of nerve tubules ; the 

 respective openings of two neighboring clusters are separated 

 from one another by a distance equivalent to the greatest 

 diameter of a cluster. Accordingly, an anterior opening is 

 separated from the next following posterior opening, and an 

 opening on the right hand from one on the left. Thus, both 

 on the dorsal as well as on the ventral side of a lateral chord, 

 there is a single linear series of cell clusters on the right, as 

 well as such a series on the left; and, consequently, also two 

 corresponding series of openings in the inner neurilemma. It 

 is the rule, further, that the cell clusters on the right hand 

 alternate with those on the left, on each side of the chord, and 

 pari passu their respective openings on the right with those 

 on the left; this alternation becomes more marked towards the 

 posterior end of the chord, where, owing to the decreasing 

 diameter of the latter, the distances between the successive 

 clusters become relatively greater. So it is seen in Fig. 25, 

 that on either side of the chord the cluster on the right does 

 not lie in the same plane as that on the left. In other words, 

 on either the dorsal or the ventral side of each lateral chord, 

 the clusters of ganglion cells II and their respective openings 

 in the inner neurilemma (these serving for the passage of their 

 respective bundles of nerve tubules into the fibrous core) are 

 symmetrically and bilaterally arranged. But this arrangement 

 does not represent a true bilaterality, since the cell clusters on 

 the right are not paired with those on the left, but rather alter- 

 nate with them. To adopt the terminology of Bateson ('94), 

 the relation of the two lateral nerve chords to one another 

 would constitute a " major symmetry " (the one being a mirror 

 image of the other) ; while that of the right and left sides of 

 each chord would represent a "minor symmetry." Fig. 26 

 represents a diagrammatic longitudinal section (in the hori- 

 zontal plane), through either the dorsal or ventral side of one 



