EMBRYOLOGY. 725 



earlier embryonic stages of the starfish. I have described in Part III. 

 the various stages of growth of the young sea-urchin, — how it passes from 

 the stage of PI. X. f. 2 to the condition when the pores, instead of being 

 arranged in single rows one above the other, are placed in arcs on both 

 sides of a median ambulacral row covered with spines. We can form a 

 tolerably accurate idea of the changes the young must pass through by 

 examining the abactinal part of the ambulacral area of an adult sea-urchin. 

 The ovarian and ocular plates are early formed round the single anal plate 

 by indistinct radiating and transverse sutures. The oldest of the young 

 sea-urchins has advanced sufficiently to enable us to see that the subse- 

 quent changes required to make it agree with its adult condition are by 

 no means as great as the changes which the young sea-urchin has under- 

 gone up to the present time. It has reached a condition which assures us 

 that we deal with a young Strongylocentrotus, and nothing else. The 

 pigment-spots, so marked in the younger stages, are smaller and scattered 

 more uniformly ; the muscular band around the mouth is well developed ; 

 the plate covering the actinal area has been separated from the edge of 

 the test, and is moved by the muscular membrane which covers the actinal 

 system. There are no notches as yet in the actinal part of the test. The 

 teeth have not changed their form from that found in the earlier stages : 

 there are from seven to eight tubercles in each vertical row of the ambu- 

 lacral and interambulacral zones. I was unable to distinguish among the 

 many tentacles the original odd tentacle which was so prominent in the 

 younger stages. Neither have I succeeded in determining the position 

 of the eye in any of the stages of these young sea-urchins, owing to the 

 early presence of the spines and of the large pigment-cells, which pre- 

 vent us from obtaining a favorable view of the odd terminal tentacle in 

 the young forms ; the odd terminal tentacle retains its original position 

 during the whole life of the sea-urchin, as is the case in the starfish. I 

 can say nothing concerning the development of additional ambulacral ten- 

 tacles, as they are covered by limestone plates ; but as in the early stage they 

 are simple loops of the main ambulacral tube, they undoubtedly increase in 

 number at the base of the odd tentacle, as in Asteracanthion (see Plate VI., 

 Embryol. Starfish). 



The figures of young sea-urchins given by Midler belong, unfortunately, 

 nearly all to different suborders from our sea-urchin (Strongylocentrotus), so 

 that we cannot make the comparison with our young sea-urchins as close 



