Echinoderm Hybridization. 135 



Fourteenth cleavage 4hoo m meridional (second 16 of eleventh to 32) 92 cells. 



Fifteenth cleavage 4 h 3 o m meridional (upper 32 of tenth to 64) 124 cells. 



511451*1 Blastulag rotating within membrane. 



6hoo m Membrane thrown off. Blastulae swim. 



5113 o m Most swimming at surface. 



8 h 25 m Mesenchyme formation beginning, 4 cells in 

 * blastocoel. 



gli25 m Beginning gastrulation. 



3 h oo m a.m. Pigment spots and skeleton beginning. 



I2 h o m noon. Many plutei with anal arms. 



The explanation of this more rapid development may be found in 

 the differences in temperature between the localities in which the work 

 has been done. 



In hybridization work it is desirable to cross forms whose plutei 

 possess striking structural differences. The most advantageous Echinoid 

 crosses upon which any detailed work has been done by the European 

 investigators arc the Sphar echinus X Strongylocentrotus and the Spha- 

 rechinus X Echinus combinations. These are advantageous because 

 the Echinus and Strongylocentrotus anal arm skeletons consist of a single, 

 slender rod, while the Splicer echinus anal arm skeleton is of the fenes- 

 trated type, being made up of three rods united with one another by 

 crossbars. The same advantage is afforded by American crosses in 

 which one of the forms used is Toxopneustes. The plutei of Toxopneustes 

 have a single rod as the support in the anal arms. The plutei of Arbacia, 

 Hipponoe, McUita, and Moira have anal arm skeletons of the latticed 

 type. 



In my observations, then, Toxopneustes will correspond to Echinus 

 (text figs, i and 2) and Strongylocentrotus (text fig. 7), while Arbacia, 

 Hipponoe, Mellita, and Moira will correspond to Sph&rechinus (text 

 figs. 3 and 6). 



ARBACIA. 



The normal Arbacia pluteus (plate i, fig. 8) is distinctly pyramidal 

 in form. The skeleton (plate i, fig. 9) is rather heavily formed, the 

 body skeleton terminating in an irregular club-like enlargement. The 

 supports of the anal arms are of the ladder-like type. I have not suc- 

 ceeded in keeping Arbacia. plutei alive in laboratory cultures for more 

 than ten days. The older plutei and young adults which have just 

 completed their metamorphosis are readily obtained in surface towings. 



HIPPONOE. 



In the Hipponoe pluteus . (plate i, figs. 3 and 4) the posterior end 

 of the body is truncated. The anal arm rods are fenestrated. The oral 

 arm rods are continued posteriorly as the dorsal body skeleton and unite 

 with the ventral body skeleton to form a basket. All of the rods are 

 heavy in character.' These plutei live well in laboratory cultures. 



MELLITA. 



The Mellita pluteus (plate i, figs. 10, n, and 12) is rounded pos- 

 teriorly. The oral arm rods are continued posteriorly as the dorsal body 

 skeleton and unite with the ventral body skeleton to form a complicated 

 basket. The anal arm rods are fenestrated. 



