PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 105 



coenosome; and successive separation of the groups occurs at the tip, the 

 medusoid of the group becoming the nectomedusoid of a new system and 

 giving rise to a new coenosome. The developing zooids on the coenosome 

 show the usual radial gradient system of buds which becomes longitudinal 

 with outgrowth. Susceptibility and reduction gradients in the subum- 

 brellar ectoderm of the fully developed nectomedusoid are bilateral, that 

 is, susceptibility and rate of reduction decrease from the side opposite the 

 coenosome to the region where it develops. This bilaterality appears as a 

 gradual change from the basipetal gradient of the earlier medusoid bud 

 and is evident before morphological bilaterality is distinguishable in the 

 developing medusoid attached to the coenosome. That side of the bud 

 toward the attached end of the coenosome becomes the high end of the bi- 

 lateral gradient; and the highest region on that side is next to the velum, 

 that is, the polar gradient is not entirely obliterated. Susceptibility and 

 rate of reduction increase with development of motility in the zooids. 

 The definite and constant relation of the bilaterality of the nectomedusoid 

 to the axis of the coenosome or stolon to which it is attached suggests that 

 it originates in a gradient along the coenosome axis. The medusoid origi- 

 nates as a bud at right angles to the coenosome axis; and the side of the 

 bud toward the attached, physiologically younger end of the coenosome 

 becomes the high side of the bilateral gradient. The polar gradient of the 

 medusoid originates from the radial gradient system of the early bud, 

 which becomes longitudinal with outgrowth. 



OTHER COELENTERATES 



Susceptibility and rate of reduction decrease basipetally in the ectoderm 

 of the Aurelia scyphistoma and the sessile scyphozoan, Haliclystus 

 (Child). A gradient in the indophenol blue reaction has also been shown 

 to be characteristic of polarities originating in reconstitution of the stalk 

 of Haliclystus (Watanabe, 1937). Oxygen uptake of exumbrella and 

 mesogloea of the scyphomedusa Cassiopea xamachana is only about 25 per 

 cent of that of the intact animal at rest; since respiration of the meso- 

 gloea is extremely low, the exumbrellar epithelium evidently has a much 

 lower respiration than that of the subumbrella. How much of the differ- 

 ence is due to absence of entoderm in the exumbrella is not known (Mc- 

 Clendon, 1917). Developmental stages of an alcyonarian "sea pen" from 

 early cleavage to the first polyp show basipetal decrease in rate of dye re- 

 duction. 



Few data concerning actinians are available. Young individuals of 



