332 Prof. E. W. MacBride. The Artificial Production of 



constitutes the madreporic vesicle, so termed because it lies just under 

 the madreporite close to the madreporic pore. In my paper ou the develop- 

 ment of Echinus esculentus (12) I put forward the view that the vesicle 

 represented a vestigial right hydrocoele. At that time I had never seen an 

 Echinus larva showing a genuine right hydrocoele. It will be made plain in 

 this paper that that interpretation was incorrect, for the madreporic vesicle 

 may be well developed in larvae which show an unmistakeable right hydrocoele 

 (figs. 18, 21). 



On the right side of the larva, about opposite the spot where the amniotic 

 invagination is. formed on the left side, just above and below the loop of the 

 ciliated band which extends between postero-dorsal and post-oral arms, two 

 knobs begin to appear about the 20th day. About the 25th day it becomes 

 obvious that each is developing into a pedicellaria. Each is based on a 

 calcareous plate -which becomes one of the five genital plates of the young 

 sea-urchin. 



As the larva grows older an additional pedicellaria and several square- 

 topped spines appear on each plate. These plates are just as characteristic of 

 the right side of the larva as the Echinus- rudiment is of the left side 

 (fig. 6) ; midway between them another calcareous plate is developed {cole, 

 fig. 6). 



We may now consider the development of the Echinus-rudiment. This 

 structure, originally small, grows until it occupies the whole side of the larva. 

 The hydrocoele gives rise to five lobes which protrude into the amniotic space, 

 each covered, of course, with a layer of ectoderm derived from the floor of 

 the amniotic sac. These lobes are the five primary tentacles which 

 constitute the radial water-vascular canals of the adult. It is characteristic of 

 Echinus miliaris that each primary tentacle soon gives rise to two lateral 

 buds which constitute the first paired tentacles, whereas in Echinus esculentus 

 this event does not take place until after metamorphosis. Between the bases 

 of each two adjacent primary tentacles there are developed as outgrowths of 

 the floor of the amniotic sac four pointed spines, each of which soon develops 

 at its base the neuro-muscular collar characteristic of the adult spine. This 

 area of the amniotic floor, between two tentacles, which gives rise to these 

 spines may be termed the interradius. From the outer wall ot the left 

 posterior coelom lying beneath each interradius, a pocket-like evagination is 

 formed. Each pocket becomes one of the dental sacs {d.s,, fig. 19) of 

 the adult and in its walls calcareous ossicles are formed which give rise to a 

 tooth and the pair of jaws embracing it. The five dental sacs and the 

 ossicles to which they give rise constitute the complex structure known as 

 Aristotle's lantern in tlie adult. After metamorphosis an outgrowth from the 



