DEVELOPMENT OF THE YOUNG. 289 



them by threads in the vessel, near the bottom, hori- 

 zontally ; with a view to obtain some of the embryos 

 rooting themselves thereon, which I might afterwards 

 take out, to watch their progressive development under 

 the microscope. Meanwhile I secured the first step 

 in the inquiry, by opening with needles some of the 

 crowded vesicles of the adult polypidom, from which 

 I obtained some of the minute white worms. In two 

 or three days I drew out the plates of glass, and put 

 them in shallow cells of sea- water, fit for the stage of 

 the microscope. I found upon them the young 

 animals in various stages. Some of the worms were 

 yet vagrant, and crawled freely about the surface : 

 others had selected their position and were adherent, 

 but still retained the power of motion, to such a 

 degree as enabled them to change their form by pro- 

 truding certain portions of their outline : others were 

 contracted into a globule fixed and changeless, with 

 the matter produced in the form of a creeping rootlet 

 (Fig. 10). 



The next stage that I observed, was that in which 

 the adherent mass had become shelly ; as I presume, 

 for the marginal portions were perfectly transparent 

 and colourless ; and the opaque granular matter had 

 retired to the centre, where, irregular in form, it had 

 given rise to a tube (Fig. 11). This tube had 

 already formed one joint: its extremity was closed 

 and rounded, and had not yet begun to dilate into a 

 cell. The medullary matter, proceeding from the 

 granular mass at the base, passed through the lower 

 portion of the tube as a central cord, but completely 

 filled the terminal moiety. Another specimen had 

 c 2 



