410 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



In Isometra vivipara the eggs are inclosed in a thin membrane, which to a 

 greater or lesser degree, but never entirely, has the apjjearance of a fine network. 



In Tro-piovietra picta the fertilization membrane is finely sculptured and closely 

 beset with small spines. 



EOOS. 



Seeliger states that the eggs of Antedon adnatica just extruded and fixed in 

 sublimate-acetic acid average 0.25 mm. in diameter, individual eggs reaching 

 0.30 mm. 



On account of the abundance of 3'olk material the eggs are very opaque, so 

 that only the earliest cleavage stages can be studied in the living object. 



The eggs of different animals differ in size, transparency, and color, but Avith 

 trifling variations all the eggs of any one individiuil are alike. 



Usually the eggs are yellowish or light reddish, tliough often cjuite white. 

 "VMien white they are almost always entirely opaque. 



Bury says that the eggs of Antedon inediterranea are 0.30 mm. in diameter. 

 Wyville Thomson gives the diameter of the eggs of Antedon bifida when fully 

 grown as 0.50 mm. 



KEOMNERATION. 



The power of regeneration is most highly developed in the comatulids, penta- 

 crinites, and bourgueticrinites, and least in the Plicatocrinidse and, so far as we 

 know, in the Holopodidse. 



In the pentacrinites it is very common in Mctacrinus, nearly as common in 

 Isocrinus, less noticeable in Endoxocr'mus, and rare in Comastrocrinus and Ilypa- 

 locrinus. 



This is not the result of a greater inherent abilitj' to regenerate in MetacHnus 

 and Isocrimts, but arises from the fact that they run up into the shallowest water, 

 for the proportion of regenerated individuals decreases rapidly with depth, regard- 

 less of species. 



Among the comatulids regeneration is very common in all littoral species, 

 but with increasing depth becomes less and less frequent. 



Broadly speaking, it appears to occur to a much greater extent in the Macro- 

 phreata than in the Oligophreata, though partially regenerated cirri have been 

 mostly recorded in the latter. 



It is quite possible that this is connected with the larger chambered organ of 

 the Macrophreata ; and it is also possible that it is the small size of the chambered 

 organ which to a large degree limits the extension of the pentacrinites and such 

 comatulids as the Thalassometrinse and the Charitometridte toward the surface, 

 since as a result of wave action breakage is most common in the littoral, and 

 without a large chambered organ a crinoid, unless unusually tough, coidd not repair 

 its injuries with sufficient rapidity to survive. 



Minckert divides arm regeneration into the following types : 



(a) Reproductive regeneration, resulting in a simple replacement of the lost 

 part. 



