252 RITTER AND CROCKER 



GENERAL ACCOUNT OF RAY MULTIPLICATION. 



Of the time of origin of A we know nothing. In our youngest 

 specimens it is as large as any of the other live. The first two 

 new rays come in in strictly bilateral order, their place of origin 

 being the interradii A I and A II ; i. e., they are on each side of 

 A, and are adjacent to I and II. As they are obviously a pair, 

 we designate them both as x, distinguishing the one on the left 

 as Ix, and the one on the right as rx (figs, i and 2). 



By examining an. eight-rayed specimen in which Ix and rx 

 are over half grown, it will be found that two more rays are 

 appearing in the interradii 1 — lx and r^^-II. These are ob- 

 viously again a pair and we may call them Ix^ and rx^. It will 

 be observed that they, again, are adjacent to I and II respec- 

 tively. This gives us the ten-rayed stage. Individuals of ap- 

 parently this stage with Ix^ and rx^ about half the size, and with 

 Ix and rx nearly equal to, the other rays, will be found to 

 possess the beginning of another pair in the interradii I - Ix^ and 

 nv^-II, or again, adjacent to I and II ; and so on (figs. 3 and 4). 



The law of ray multiplication that is followed from the six- 

 rayed condition onward is now manifest. New rays are pro- 

 duced in pairs, lx°- and r;v", in the interradii, I-/.v''~^ and 

 rx^~^-ll; i. e., always adjacent to I and II, with the plane 

 A IV as an axis of symmetry. Or we may express the fact by 

 saying that there is a budding area adjacent to I and II on the 

 side toward A which continues to give origin to new rays until 

 late in adult life. 



To this law of ray production we have found no exception. 

 In specimens with regenerating rays some difficulty may be 

 experienced in distinguishing these from new ones not yet fully 

 grown. But in almost all such cases careful examination re- 

 moves all doubt. 



The first evidence visible to surface examination of the com- 

 ing in of a new ray is the proximal pair of ambulacral feet. 

 The new rays grow rapidly and rarely are more than the two 

 youngest pairs smaller than their fellows (figs. 3 and 4). 



One member of a pair frequently appears slightly earlier 

 than its mate, or one may grow somewhat more rapidl}' than 

 the other ; but all deviations of this sort fall readily within the 



