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 appearance of a fine network. 



In Tropiometra picta the fertilization membrane is finely sculptured and closely 

 beset with small spines. 



EOQS. 



Seeliger states that the eggs of Antedon adriatica 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 yolk 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 with 

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



Usually the eggs are yellowish or light reddish, though often quite white. 

 When white they are almost always entirely opaque. 



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

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

 grown as 0.50 mm. 



REGENERATION. 



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

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

 Imow, in the Holopodidse. 



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

 Isocrinus, less noticeable in Endoxocrinus, and rare in C ' omastrocrinus and Hypa- 

 locrinus. 



This is not the result of a greater inherent ability to regenerate in Metacrinus 

 and Isocrinus, 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 Charitometridse 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, could 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. 



