470 Mr. D. W. Devanesen. Development of the Calcareous 



urchin. The shell breaks away leaving the lantern in the centre. By further 

 manipulations with the mounted-needle the teeth were separated, dried, and 

 mounted in xylol-balsam. An examination of whole embryonic teeth 

 revealed several facts which were hitherto unknown and which threw fresh 

 light on their morphology. 



3. The Jaws. 



In describing the development of the lantern ossicles, it is perhaps well to 

 begin with the jaw and proceed upwards. Each jaw, or a pair of "alveoli," is 

 developed from a pair of tri-radiate spicules deposited in an inter-radius of 

 the " echinus-rudiment " of the metamorphosing pluteus ; two rays of each 

 rudiment of an alveolus are found embracing a tooth, one on the inner side 

 and the other on the outer side, while the third ray is seen to stretch aborally 

 and towards the rudiment of an " epiphysis " in the contiguous radius (Plate 11, 

 fig. 1, R.A.). The next step in the growth of these spicules consists in the 

 branching of the three rays at their ends and in budding new off-shoots over 

 their body ; these subsequently join end-to-end. By repetitions of this process, 

 there results a perforated ossicle having more or less the shape of an adult 

 alveolus. In fact, the rudiments of the alveoli, like all other elements of the 

 lantern, with the exception of the teeth, follow the usual method of ossicle 

 growth among echinoderms ; but, as compared with the growth of the 

 epiphyses and the rotulre, for instance, these grow mainly along their vertical 

 axes. This fact enables one to account for the great height of the lantern in 

 urchins generally. This vertical growth of the jaw has caused the ossicles 

 above to be pushed farther and farther away from the oral region. In the 

 very early stages, as may be expected, one finds all the rudiments of the 

 lantern occupying a comparatively low position. 



The upward and radially-directed growth of each alveolus results in a 

 junction between it and the rudiment of an epiphysis in the adjoining radius. 

 Each jaw, therefore, bears on the top two epiphyses belonging to different 

 radii. 



4. The Epiphyses. 



One finds their earliest traces as pairs of triradiate spicules situated in each 

 radius of the " echinus-rudiment," far below the rotulae, and above the level 

 of the jaw -rudiments. Each spicule grows by the usual method, and when a 

 more or less rectangular plate is formed, each epiphysis gives off from its 

 outer corner, in the proximity of a tooth, a process ; the two processes 

 belonging to a pair of epiphyses diverge from one another (Plate 14, fig. 5, O.E.). 

 One outgrowth meets, over the " foramen magnum " and on the outer side of a 

 tooth, another — I say " another " advisedly, for it is not its fellow — belonging 

 to an epiphysis of a different but adjoining pair (Plate 15, fig. 6, O.E.). Both 



