STUDIES IN SPICULE FORMATION. 35 



(Study IV). Wliy iu Ophiuroids (and all other echinoderms 

 save holothuriaiis) there should thus exist this definite relation 

 between the number of attached scleroblasts and the size of 

 the spicule, and not in holothurians I am quite unable to 

 say. (As a remarkable illustration of this fact compare the 

 stool spicules of Thyone [Study IV, fig. 55] with the young 

 spines of Ophiothrix [fig. 13]). 



Not all of the spicules are derived from an original ti'i- 

 radiate structure; occasionally the spherical granule elon- 

 gates to form a rod. which bifurcates at its extremities, as in 

 Cucumariidae ; occasionally also the young spicule assumes a 

 more or less irregular form, and in Ophiothrix I have ob- 

 served four- and six-rayed young spicules (figs. 10 b and c), 

 but all these are very rare. The triradiate form of the 

 young spicule is by far the most usual. It must also be 

 mentioned that the largest of the perforated plates figured is 

 small when compared with most of the spicules present in 

 the young embryos of fig. 1, and of course the plates of the 

 adult Amphiura are still larger by comparison. 



The scleroblasts in Amphiura are spherical cells which 

 assume a subspherical form when attached to the spicule ; 

 the nucleus is relatively large, and if stained with picro- 

 carmine shows a distinct nucleolus. The cytoplasm is 

 faintly granular. Nuclei vary in size to some extent even in 

 the same individual, and, as already stated, become very 

 large previous to division. In specimens fixed with absolute 

 alcohol the nuclei appear somewhat contracted. 



The Spicules in Ophiothrix feagilis and Echinus 

 esculentus. 



My material for ascertaining the mode of spicule forma- 

 tion in these two genera being very limited I have merely 

 attempted to confirm the supposition that the process is here 

 identical with that in Amphiura elegans, and this I have 

 had no difficulty in doing. My Ophiothrix material con- 

 sisted of several young specimens with discs about 1 ram. in 



