GEORGE B. CHAPMAN 135 



periodic structure is so rarely observed. It may be, however, that the 

 preparative treatments usually fail to preserve this feature of the 

 structure. Figure 4 also includes a section through the enlarged 

 head of the tubule ( HD ) . 



Figure 6 includes portions of two interstitial cells (I) and a 

 cnidoblast with a nearly mature stenotele. The cell at the lower left 

 is considered to be the least differentiated of the three cells, using 

 the criterion of sparse endoplasmic reticulum, as suggested by Slaut- 

 terback and Fawcett (12). That cell is of great interest, however, 

 because of the presence of a centriole ( CE ) with radiating spokes, 

 a configuration reminiscent of, yet somewhat different from, that 

 described in flagella by Gibbons and Grimstone (5). Although the 

 angle of section through the cnidoblast is inappropriate, the centri- 

 ole in the interstitial cell may be seen to bear a similarity to the 

 section through the base of the immature cnidocil ( CD ) . This 

 figure is, then, considered to indicate a relationship between the 

 centriole and cnidocil. Thus, the relationship between centrioles and 

 cilia and flagella may be extended to include cnidocils. ( More 

 extensive evidence in support of the differentiation of centrioles 

 into cnidocils will be presented elsewhere. However, it should be 

 noted here that Bouillon ct al. (2) believe the cnidocil to have a 

 structure anatomically distinct from that of cilia and flagella.) 

 Several elements of the outer supporting structures ( SP, ) and 

 inner supporting structures (SP^) of the cnidocil, as described by 

 Chapman and Tilney (3), are also visible in this figure. 



Figure 7, an oblique section through the apical region of a 

 stenotele and a nearly transverse section through the cnidocil, also 

 reveals the similarity between the centriole and cnidocil base. Al- 

 though the state of preservation of this particular cnidocil renders 

 extensive discussion unwarranted, it may be stated that there ap- 

 pear to be nine peripheral groups of filaments, at least two of 

 which are composed of two elements, and a central group of three 

 filaments. In the original print, three circular profiles can just be 

 distinguished in the core of the cnidocil, once again reminiscent 

 of an appearance seen in flagella by Gibbons and Grimstone (5). 

 It should be noted that, even in their extensive study of flagellar 

 structure, Gibbons and Grimstone (5) apparently did not encount- 



