142 C. M. CHILD. 



more rapid staining of the subumbrella than of the exumbrella 

 like other hydromedusse, but also a beautiful and striking bilater- 

 ality in the staining gradient, the side opposite the origin of the 

 colony stem staining most rapidly, the stem side least rapidly. 

 This difference is also visible with susceptibility methods, but 

 less distinctly. In the groups of zooids on the stem each zooid, 

 both nutritive and medusoid, shows a basipetal color gradient 

 and the medusoid zooids also show a bilateral gradient similar 

 to that of the nectocalyx. In fact, these medusoids along the 

 stem may later become nectocalyces, for each group, consisting 

 of one nutritive, one medusoid zooid, a group of tentacles and a 

 bract, may become free and develop into a new siphonephore 

 colony. The bilaterality of the color gradient in this form is a 

 very striking feature. 



Young actinians including the genus Peachia which is parasitic 

 on hydromedusae during its earlier life-history, and several other 

 undetermined forms showed a very distinct basipetal color 

 gradient in tentacles and body, and with certain concentrations 

 a similar disintegration gradient. 



In various embryonic and larval stages of polyclads, a beautiful 

 basipetal color gradient appeared uniformly. 



Unfertilized eggs of the sea urchin, Strongylocentrotus fran- 

 ciscamis show uniformly a very definite color gradient, although 

 in the absence of definite landmarks it is not absolutely certain 

 that this gradient is basipetal. However, the egg after fertiliza- 

 tion and during cleavage does show a distinct basipetal gradient, 

 even in each of the first two blastomeres, and these can be little 

 doubt that this gradient is identical with that of the unfertilized 

 egg. This gradient is also present in blastula and gastrula and 

 is identical with the susceptibility gradient (Child, 'i6a). 



The elongated tentacles and the branched branchial apparatus 

 of the annelid, Amphitrite both exhibit very marked basipetal 

 color gradients and in tadpoles of the solitary ascidian, Corella 

 willmeriana the growing tail stains most rapidly at the tip with 

 a definite gradient to the base, and the three papillae of attach- 

 ment which in this form grow out into temporary stolons during 

 metamorphosis, but are later resorbed, are also regions of rapid 

 staining. On the body of the larva staining progresses from the 

 papillae more rapidly on the dorsal than on the ventral side. 



