f)8 



The ijaslrulalioii. which proceeds in the iisiinl \v;iy (i'l. W'll. I'"i.i>s. 1 2) 

 hci^iiis ;il liic age of aboiil 18 hours. Tlie blaslo|)oriis is soiuelimes pushed 

 oul towards llie side (PI. XVII. Fii^s. 2 — .'J); this is. however, hardly any- 

 Ihiui* bul an iu(n\i(hial variation; generalls il is in Ihc middle of the 

 hind end of the i)ody. and in this place a trace of il is ••enerally found 

 in the more advanced stages (Fl.XVII. Figs. 5 — (i; Fl. XVllI. Fig. 7). The 

 gastrnla' are not i)ound to the surface, but swim freely in the water, witii 

 the usual rotating movement, the aboral vn(\ always being turned up- 

 wards, as a necessary consequence of the fad Ihal the vacuoles containing 

 the light. |)r()l)al)ly fatty substance art' in Ihe main conruicd lo this p;iil 

 of liu' body, and il remains so during llic whole course of Ihe develop- 

 menl. II seems bevond doubt that tiie substance conlaint'd in these 

 vacuoles forms the nourishment on which (he embryo subsists until the 

 young sea-urchin has got its mouth and is able lo feed actively; the 

 embryo itself is perfectly unable to feed, the gastrula mouth 

 closing very early, sometimes already at the age of oidy .'50 hours (PI. X\'I1. 

 Fig. 7) and no larval mouth being formed. 



In embryos 30 hours old thearchenleron begins to dilTerentiale (PI. XVII, 

 l''igs. ."J- -4), its u])pei' end widens, anfl gradually the widening pushes itself 

 downwards, the wall of the archenteron making a fold on one side. The 

 pouch thus formed, representing the rudiment of the hydrocoel, is still 

 in open connection with the archenteron. bul in embryos of 12 houis il 

 has been separated from it (PI. XVII. Fig.6; PI. XVI 11. l-'ig. 7). The forma- 

 tion of the enlerocoel cannot be made oul in the material available, .lud- 

 ging from this material it would a|)pear to originate as schizocoel spaces 

 in, not as pouches from the entoderm; but 1 do not venture lo ascertain 

 as a fact thai it does originate in this exceptional way. A rich and 

 most perfectly preserved material would be needed for giving full evidence 

 of such a surprising statement, and my material cannot be said to be of 

 such eminent quality. Also the question of the formation of the hydro- 

 pore must remain unsettled; in fact, 1 have been unable lo find the 

 slightest trace of il, and it would appear that also in this regard Ihe 

 development of this species is exceptional among luhinoderms. (Clomp, 

 below, sub Peronella Lesueuri). 



As already stated the blastoporus may close com|)letely already a I the 

 age of .30 hours (PI. XVII. Fig. 7), the archenteron com|)letely .separaling 

 from the ectoderm. In most cases, however, the separation is not com- 

 plete, a fine strand remaining as a connection between the enlodeiin and 

 the ectoderm (PI. XVII. Figs. 5 — (i). Within the cavity of the archenteron 

 a distinctly limited mass of a substance is generally seen which stains vei\ 

 strongly in hematoxylin or safranin; it may perhaps be an agglutinated 



