RAYS AND BILATERAL SYMMETRY 



259 



means in contact, but are separated by some quantity of differ- 

 entiated, sustentative, or skeletal tissue. 



It will be remembered that the independent parts which con- 

 tribute to the production of the new ray are : the body coelom, 

 the water ring canal, the outer perihasmal canal, the ectoderm 

 with the ring nerve, the interradial mesenchyme, and, finally, 

 the digestive tract. 



From the description it is obvious that all these, except the 

 last, start off almost simultaneously toward the production of the 

 new ray. Is this start absolutely simultaneous, or does some 

 one, and if so which one, lead the way? 



We have given considerable time to the effort to answer this 

 question, but have not been altogether successful. The evidence 

 that we have is to the effect that the water ring canal leads off 

 in the development. Fig. 13 represents an interradial section 

 in the interradius in which a new ray should develop. The 

 orientation of this section will be understood by noting the ring 

 nerve band, «.r., the peristome -per.^ and the body coelom, coe.e. 

 The water ring is seen at w.v.r. The great thickening on the 

 wall of this at r.w.v' . is probably the very beginning of the 

 radial water vessel of a new ray. 



The definite circumscription of this patch of cells, indicating 

 that it has already begun to evaginate, and the form and stain- 

 ing character of the individual cells leave littfe doubt that it is 

 the beginning of a definite structure of some kind. Its posi- 

 tion both as regards the interradius in which it occurs, and on 

 the outer side of the canal is strong evidence that it is the 

 radial canal. And this presumption is strengthened still fur- 

 ther by the fact that no other organ develops from the ring 

 canal in this locality. If it is not the beginning of the radial 

 canal it is an abnormal growth, and this is improbable. If 

 this is what we suppose it to be, the initial step in the pro- 

 duction of the new ray is taken in a very positive way by 

 the water ring ; for no evidence of new growth can be detected 

 at any of the points where the other elements of the ray would 

 begin. The coelomic epithelium of the new ray would begin 

 about at the point jy, and the ectodermal invagination at x^ 

 which corresponds to a- in figs. 9 and 10. We should rather ex- 



