SPICULES OF SYNAPTA AND AURICDLAKIA LARVA. 507 



EXPLANATION OF PLATES 29 & 30, 



Illustrating Mr. W. Woodland's " Studies in Spicule Forma- 

 tion. VII. — The Scleroblastic Development of the Plate- 

 and- Anchor Spicules of Synapta, and of the Wheel 

 Spfcules of the Auricularia Larva," 



Fij^s. 1, 9-25 X 800 diameters; Figs. 2-S, 20-12 x IGOO diameters. 



PLATE 29. 



The Development of the Synapta Piate-aiid-Anchor Spicules. 



Pig. 1.— Portion of a transverse section through the body-wall of Synapta 



nhserens showing tlie multiplication of nuclei at one centre on the internal 



side of the dermal epithelium to form a syncytial mass. To the right lies the 



circular muscle layer and between this and the dermal epithelium some muscle- 



and nerve-fibres. 



Fig. 2. — A syncytium viewed from the internal side of the body-wall. The 

 initial granule is deposited on its internal aspect — i. e. the majority of nuclei 

 are situated on its external side. These syncytia have very definite outlines, 

 being quite distinct and usually distant from all surrounding structures. It 

 must also be remarked that, owing to difKcullies of observation, the exact 

 number of nuclei figured cannot be guaranteed as having been the actual 

 number present. It can, however, be guaranteed that every nucleus figured 

 was carefully observed and correctly placed relative to the spicule, but, as is 

 self-evident, it is impossible to be certain that every nucleus i)reseut was 

 observed. 



Fig. 3. — The granule has elongated slightly towards the observer's left. It 

 is possible that the syncytium is in part formed by the fusion of at-first- 

 separate scleroblasts as well as by the multiplication of nuclei; this latter, 

 however, is the principal mode of formation of the syncytium. Some of the 

 nuclei are larger than others, a feature probably denoting approaching nuclear 

 division. 



Fig. 4. — The initial granule has here elongated to the observer's right hand. 



Figs. 5 — 8 illustrate the further elongation of the granule to one side or 

 the other. The axis of the future shaft is first discernible when the full length 

 is nearly attained. The majority of the nuclei, it will be noticed, lie on the 

 external side of the shaft. 



Fig. 9.— The nearly adult shaft. 



