440 



EMBRYOLOGY 



tions are developed on the umbrella of the Pluteus, one on 

 the dorsal surface and one on the anal area. Each of these 

 soon assumes a trilobed form, and they prove to be the two 

 first pedicellaria of the sea-urchin. Fig. 214 shows this 

 condition in another sea-archin {Arhacia pustulosd) } With 

 the progress of development the disc continually increases 



Fig. 213. 



Fig. 212. 



Figs. 212 and 213.— Parts of a Spatangoid Pluteus (after Mktschnikoff). D, 



intestine; Ei, invagination of the larval skin, which in Fig. 213 covers the 



hydrocoele (if). The latter opens to the exterior by means of the dorsal pore. H, 

 hydrocoele ; P, dorsal pore ; Sfc, larval skeleton. 



in circumference, and at the same time the opening of the 

 invagination also widens again. The contractile feet are 

 finally extruded through the latter, and are now seen to 



1 [Very thorough and accurate investigations of these features of de- 

 velopment have recently been carried on by Theel (No. XXIX.) on 

 Echinocyamus, a form in which the larval development and the meta- 

 morphosis could be established almost without a gap. Unfortunately 

 his fine results cannot be stated here in brief form, and we are compelled 

 to refer to the original paper. — K.] 



