REPORT OF THE DIRECTOR I9IO 233 



not possible, but its width suggests the presence of a third b.v. 

 The measured length of the upper right-hand sutures of the IBB 

 is as follows: 1. post. IB 2.2 mm; r.post.IB 2.2 mm; r. ant. IB 2.0 mm; 

 ant.IB 1.7 mm; 1. ant. IB 1.6 mm. An additional reason for the 

 choice of the vertical suture by our aberrant b.v. 9 may be here 

 seen, as what should have been its normal suture offered it the least 

 room for the exercise of its function. 



The oral radial group. Text figure 12 shows a triangular area 

 of b.vs. developed around the aborad apex of l.ant.R. In order to 

 avoid confusion we have used letters to designate the members 

 of this group. As these were developed at the upper right-hand 

 corner of l.ant.B., their places of origin on the plate must occur 

 along a line connecting the primitive plate corner w'ith the pres- 

 ent plate corner. This line is represented by a series of ten 

 equidistant and numbered dots. The upper left-hand portion of 

 l.post.B. has been treated in the same manner and it must be seen 

 that the position of any dot of either series, w^as, at some time 

 during growth, identical with that of the similarly numbered dot 

 of the other series. B.v. a did not appear until the plate had 

 attained about one-third of its present diameter, or until the plate 

 corners had reached the position of dot 4. B.v. d appeared when 

 the plate corners had reached dot 7 and b.v. "■ when the plate cor- 

 ners had reached dot 10. The extension of b.v. e suggests dot 8, 

 while / would correspond well with dot 9. This indicates a 

 counter-clockwise rotation for this group. 



^^'e must note that the correspondence is not exact and exact- 

 ness should not be expected where rate of groAvth is compared 

 with a hypothetical mathematical schedule. The diagram, how- 

 ever, contains some errors. First its dotted " lines of origin " are 

 a little too long and bring b.v. i of the aborad radial group in line 

 with the upper ends instead of the low^er ends of the common 

 suture of the BB. Second, the line should be a curved line and 

 not a straight one as the plate is not flat but convex. Equidistant 

 dots on a curved surface would no longer be equidistant when 

 reproduced on a photograph of that surface. Third, the plate 

 center is in itself a point which can not be located with exactness. 

 This form of diagram has been used for its suggestiveness and its 

 simplicity. Another manner of approach would have been 

 through extending b.vs. a, d and g to the '' line of origin " and 

 then dividing these portions of the line into three equal parts. 



We may, however, strengthen our suggestion of counter-clock- 

 wise rotation for this group by using the measured distances of 



