EMBRYOLOGY OF ECHIXODEkMS. 99 



XIY. THE EMBRYOLOGY AXD ^METAMORPHOSIS 

 OF ECHIXODERMS. 



The eggs of the Echinoderms are especially adapted 

 for examination by a beginner, on account of the sim- 

 plicity of the early stages ; and the student of the elements 

 of morphology can nowhere find more favoi-able material 

 for studying the more general features of embryology. 

 The eggs of Arbacia are in certain respects unfavorable 

 for the study of the special features of echinoderm embry- 

 ology', but the ease with which they may be procured and 

 reared, and the fact that the breeding season extends 

 through the whole summer, render it, on the whole, the 

 best form for our purpose. 



Those who wish to pay more extended attention to the 

 subject may study the eggs of Strongylocentrotus, in con- 

 nection with those of Arbacia ; for while the opacity of 

 the latter renders the ol^servation of tlieir internal struc- 

 ture very difficult, the eggs of Strongylocentrotus are 

 transparent. The excellent figures, by Alexander Agas- 

 siz, of the metamorphosis of Strongylocentrotus, have 

 been reproduced in Agassiz' Seaside Studies, Packard's 

 Life Histories, Balfour's Comparative Embryology, and 

 other text-books ; so that the student can readily obtain 

 from them such guidance as he will need for more ex- 

 tended research. 



I. The fertilization of the eggs of Arbacia. The 

 spawning season of this species on the southern coast ex- 

 tends from early spring to the end of August, and on the 

 northern coast it probably lasts several weeks longer. 



The eggs may be oljtained by chopping up the ovaries ; 

 or they may usually be obtained after they have been laid. 



