EMBRYOLOGY. 



The embryos of our common sea-urchin* resemble most closely, in some of 

 their earlier stages, those of Strongylocentrotus lividus, figured by Muller on 

 Plates VI. and VII. of his fourth Memoir.t The figures which Muller gives 

 correspond with what 1 have observed in larvae obtained by artificial fecun- 

 dation. He succeeded in tracing them for about three weeks, which is not 

 quite as long' as 1 have kept them alive. Muller has unfortunately not given 

 us any figures of the very first stages of this species, nor has he found the 

 adult larvae swimming about immediately before the resorption of the plu- 

 teus. The series of figures found in the Memoirs of the American Aeademy,$ 

 and here reproduced, will give us a more complete idea of the different phases 

 of growth of one species of Echinus than can be gathered from a comparison 

 of the different species which Midler has investigated. It will enable us 

 to trace the order of appearance of the anus of the pluteus, and the last 

 changes which the larva undergoes immediately before the young sea-urchin 

 has resorbed the whole framework of the pluteus. 



The progress of segmentation of the yolk is entirely similar to what we 

 observe in the starfish ; the main differences in the eggs are simply of propor- 

 tion between the relative size of the yolk-mass and the outer envelope. My 

 observations agree with the account of the segmentation given by Derbes. § 



In addition to the striking difference in color noticed between the sper- 

 maries and ovaries at the time of breeding (the latter are yellowish-orange, 

 while the former are milky-white), we find external differences at that period 

 which enable us readily to distinguish the males and females by the more 

 vivid coloring of the spines of the latter, which are of a violet tinge, while 

 those of the males are more yellowish-green. 



The earlier stages of development here described were raised from eggs 

 artificially fecundated, while the older pluteus stages were found nomadic, 



* Strongylocentrotus Drobachiensis. 



t Muller, J., Ueber die Larven und diu Metamorphose der Eckinodermen. Vierte Abhandlung. 

 Berlin, 1842. 

 I A.GASSIZ, Alex., Embryology of Ecfainoderms, Mem. Am. Academy, IX. 1864. 

 § Derbes. Ann. d. Scien. Nat. 3" Serie. VIII. p. 80. 1847. 



