ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 283 



below. This was probably due in large part to the relative unripeness of the sex 

 cells. 



In 64 hours most of the larvae were pigmented gastrulse. The star-shaped skeletal 

 element was well advanced. Some few were assuming the form of the prism larva. 



4 days. The larvae were early plutei with the postoral arms well advanced, antero- 



lateral arms commencing, and numerous spines on the apical rods. 



5 ,, Antero-lateral arms increasing. Pigmentation slight. Very varied rate 



of advance. 

 7 „ Apex dome-shaped, arms rather long and slender. Pigment light and 



scattered in dots and stippled lines, not in definite patches. This seems 

 to be typical of E. acutus. 

 11 ,, Postero-dorsal arms just commencing. 



14 ,, Skeletal element of preoral arms appeared. 

 17 „ Pre-oral arms commenced to grow out. 



21 ,, 8-armed pluteus. Commencement of anterior ciliated epaulettes and of 



thickenings to form posterior ciliated epaulettes. 

 26 „ Appearance of £'c/imM.-.-rudiment and formation of posterior ciliated 



epaulettes. 

 3?) ,, This example, although seven days older than the preceding, had not yet 



developed an Echimis-rud'iment. 

 42 ,, Both anterior and posterior ciliated epaulettes completed. Lateral and 



posterior pedicellariae. 



(3) A', miliaris % X E. miliaris $ . {Cf. Plate 19, figs. 19-25.) 



This culture progressed very rapidly. This was not due alone to the ripeness of 

 the sperm and ova, for this cross always develops quickly. 



45 hours. Prism larvae. This was earlier than in any of the other cultures 

 described. Shortly afterwards the commencement of the antero-lateral 

 and postoral arms appeared. 

 5 days. Plutei with pointed posterior ends, which, in side view, resemble conical 

 caps. Apical rods smooth and in the forni of thick clubs. Pigment 

 scanty and in dots. The larvae were very transparent. 



10 ,, The postero-dorsal arms were well in evidence. Commencement of the 



preoral arms. 



15 ,, Appearance of the iiWimMs-rudiment. 



20 ,, The ciliated epaulettes were established, with the two pairs of green 



pigment masses, typical of this species. 



There was also some green pigment in the arms. Most specimens 



had a pair of lateral pedicellariae opposite the Echinus-rudiuient. Some 



of the plutei had, however, not yet developed either ciliated epaulettes 



or Echinus-rudiment. 



2 o 2 



