70 



jHiil ullimaU'ly llic embryo splits open ht'iT aloiif* Ihc whole ventral side. 

 At lirst the tuheleel and the spines are arran<>ed in a sinf>le row in the 

 rather nairow slit (PI. W'll. Fig. 10), but gradually the slit opens more 

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

 or less distinct circular arrangement (IM. W'll. i-'ig. 1 1). and nllimately 

 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 anniiolic 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 cree|)s out ol the embryo as of an egg-shell, carrying now the 

 rests of it on its back (f^l. XVII. Fig. 9, 12). 



.\11 the processes of transformation connected with the metamorphosis 

 are limited 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, (iradually, as the food is absorbed, the "wallet" shrinks 

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

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

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

 part ol I lie embryo "gradually becomes completely overgrown by the 

 young urchin and enclosed within its l)ody". Tliis expression is not (piite 

 correct; the aboral part is not overgrown by the sea-urchin but sim|)]y 

 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. 



As already slated, 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 oidy 

 be mentioned that the lirst spines to appear are of the trifid. embryonal 

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

 Echinoids. 



