10 THÉEL, DEVELOPMENT OF ECHINUS MILIARIS L. 



Explanation of the plates. 



The plates exhibit a selection of figures drawn with the aid of 

 Abbe's caraera lucida from transverse sections across the outgrowing 

 sea-urchin in three dififerent developmental stages. Consequently, the 

 numbers of the figures indicate the nuraeration in the coraplete series 

 of drawn sections which are to be published låter on. 



Digestive organs of the larva are thinly lined. 



Hydrocoele is closely lined. 



Left posterior coelom is black. 



Ectodermic invaginations or tissues are dotted. 



Wander-cells are marked by roughly wrinkled configurations. 



a,, araniotic cavity. 



&., buccal membrane with its cavity. 



h.c, buccal cavity. 



e., ectoderm of the Pluteus. 



e.c, epineural canal or radial opening between the buccal and 

 araniotic cavities. 



e.d., ectodermic disc. 



/.c, fold of the left posterior coelom entering the buccal membrane. 



i.e., interradial part of the ectoderm. 



e./.c, interradial folds of the coelom becoming the pharyngealsinus 

 etc. in the future Echinus. 



i.n., intestine of the Pluteus. 



i.o., interradial opening into the cavity of the buccal membrane. 



;"., jelly-like tissue with scattered wander-cells. 



l.p.c, left posterior coelom. 



71., nerve-ring. 



n.c, nerve-cord. 



o. opening between amnios and the exteriör, where the primary 

 ectodermic invagination has taken place. 



oe., oesophagus. 



p., pediccls or secondary feet. 



p.t.e., primary tube of the ectoderm. 



p.t.h., primary tube of the hydrocoele constituting, together with 

 the former, the tube-foot. 



r.c, ring-canal of the hydrocoele. 



r.c.h., radial canal of the hydrocoele. 



.s'., stone-canal. 



