MORPHOLOGY OF THE MADREPORARIA. 89 



trea. The larval polyps were not reared beyond the completion 

 of the first two cycles of mesenteries and the first three cycles of 

 septa (two cycles of entosepta and one of exosepta). On any 

 colony, however, are many developing polyps which present 

 intermediate stages between the commencement and completion 

 of the third cycle of mesenteries and the fourth cycle of septa ; 

 and from these results have been obtained supplementary to 

 those already presented. Mature polyps of 5. radians have 

 rarely more than three cycles of mesenteries and four cycles of 

 septa (three inner cycles of entosepta and an outermost cycle of 

 exosepta). 



First, it may be ascertained what are the relationships between 

 the third cycle exosepta of Fig. 6 and the third cycle entosepta 

 and fourth cycle exosepta found in the mature corallite. By- 

 means of serial sections through decalcified immature bud polyps 

 it has been possible to establish the relationship of these with 

 respect to one another and to the new mesenteries of the third 

 cycle. The results are diagrammatically represented in Fig. 7 

 (a g), the complications due to the presence of synapticula 

 being omitted. Fig. a represents a section through an exoseptum 

 of the third cycle, corresponding to one of the exosepta in Fig. 6, 

 only that the peripheral extremity is now bifurcated. It is in the 

 same stage as each of the four exosepta of the second cycle in 

 Fig. 4, but is shown united with the calicinal wall, and no mesen- 

 teries have as yet appeared within the two limbs. Fig. b shows 

 the same septum taken from a section at a higher level. Within 

 the exoccelic bifurcation there has now appeared a pair of third 

 cycle mesenteries, in every way comparable in their relationships 

 to the third cycle exosepta with those of the second cycle mes- 

 enteries to the second cycle exosepta of Fig. 4. 



Fig. c, from a still higher level of the polyp, reveals a further 

 stage. Within the entoccele of the mesenteries is seen the rudi- 

 ment of a third cycle septum. The latter is a new formation, 

 comparable with the rudiments of the second cycle entosepta 

 shown in Fig. 5. One limb of the exoseptum has also become 

 free. 



Fig. d, taken from a section above that of Fig. c, shows the 

 newentoseptum becoming larger, and extending centrally further 



