TOO 



SIDERASTREA RADIANS. 



.Ill* 



III2 



IIIJ 



Ills 



ins 



III3 



II 



When describing the septo-mesenterial relationships in the mature 

 polyp, it was found that the fourth-cycle septa occur within the exocoeles 

 between all the adjacent pairs of mesenteries. On leaving the last section, 

 however, the third cycle was constituted of exosepta ; only the first and 

 second cycles were entosepta ; therefore, as in the case of the secondary and 



tertiary cycles, the problem is first 

 to determine the morphological re- 

 lationship of the tertiary exosepta 

 of the developing polyp to the 

 tertiary entosepta, and also to the 

 quaternary exosepta of the mature 

 polyp. Do the tertiary exosepta 

 at the stage where only second- 

 cycle mesenteries are present be- 

 come the tertiary entosepta when 

 the third-cycle mesenteries appear, 

 or are they continued as the 

 fourth-cycle exosepta, in which case 

 the tertiary entosepta arise de 

 novo ? 



The relationships to be determined have been studied by means of 

 serial sections of nearly mature decalcified polyps. A complete series of 

 stages, representing the development of a pair of third-cycle mesenteries 

 along with the associated septa in their relation with the older mesenteries 

 and septa, is given on plate 9, figs. 54-60. The figures are taken from a 

 series of transverse sections of a retracted polyp, and will be described as 

 seen in passing from the lower stomodaeal region to the uppermost margin 

 of the calice and polyp. On account of the presence of synapticula and the 

 peripheral resorption of the mesenteries, certain complications are introduced 

 which render a clear conception of the stages somewhat more difficult than 

 would otherwise be the case. The diagrammatic figures on p. 103 will assist 

 in following the description of the sections. 



The polyp from which the drawings were made is that of which the 

 mesenterial plan is diagrammatically represented in fig. n, p. 100. Within 

 the ventral system on the left side occurs a pair of small third-cycle mesenteries 

 (lll 3 ), situated on the dorsal aspect of a pair of second-cycle mesenteries 

 (11). Plate 9, figs. 54-60, represents the members of this ventral system 

 at different levels, and the mesenteries and septa are indicated by the same 



Fie. ii. Diagram of mesenteries in the polyp from which figs. 

 54-60, on plate 9, were taken. 



