70 



and ultimately the embryo splits open here along the whole ventral side. 

 At first the tubefeet and the spines are arranged in a single row in the 

 rather narrow sUt (PI. XVII. Fig. 10), but gradually the slit opens more 

 and more, giving room to the tubefeet and spines, which assume a more 

 or less distinct circular arrangement (PI. XVII. Fig. 14), and ultimately 

 the slit becomes so wide that the upper and lower ends of the embryo 

 are pushed up on the back of the urchin; the small part left untouched 

 by the amniotic invagination acts by this process as a hinge between 

 the upper and lower parts. In fact, it has the appearance that the young 

 sea-urchin creeps out of the embryo as of an egg-shell, carrying now the 

 rests of it on its back (PL XVII. Fig. 9, 12). 



All the processes of transformation connected with the metamorphosis 

 are hmited to the very small part just below the widening of the em- 

 bryonal body. The large aboral part remains perfectly passive during the 

 whole process of the development from the gastrula stage unto the 

 metamorphosis. It is simply a reservoir of food to the embryo, and the 

 metamorphosing sea-urchin thus literally carries a wallet along with it 

 on its back. Gradually, as the food is absorbed, the "wallet" shrinks 

 (PI. XVII. Figs. 12—13; PI. XVIII. Figs. 8—9) and finally it disappears 

 completely, the young sea-urchin at the same time assuming its regular 

 shape. — In the preliminary notice (p. 205) it is stated that the aboral 

 part of the embryo "gradually becomes completely overgrown by the 

 young urchin and enclosed within its body". This expression is not quite 

 correct; the aboral part is not overgrown by the sea-urchin but simply 

 absorbed, and its skin becomes directly part of the skin of the young 

 urchin. 



The sections of the metamorphosing sea-urchin represented in PI. XVIII. 

 Figs. 8 — 9 show the stone canal and its outer and inner opening, the 

 body cavity etc. ; but there is no reason to enter on a detailed description 

 of all these various, structures, since it has not been possible to trace 

 them from their first origin. The character of the aboral part as a simple 

 food reservoir, containing no special structures, comes out very distinctly. 



jAs already stated, the calcareous structures have been completely dis- 

 solved in all the specimens; accordingly no information can be given of 

 the development of the skeleton of the young sea-urchin. It should only 

 be mentioned that the first spines to appear are of the trifid, embryonal 

 type, which appears to be of general occurrence in the young of all regular 

 Echinoids. 



