108 Dr. J. E. Duerden on the 



The musculature is strongly developed througliout, whether 

 the mesentery be simple or divided. 



From the series of sections it appears that the two con- 

 necting mesenteries, which originally pass straight across the 

 portion of the disk intervening between the two oral apertures, 

 first begin to extend from their middle towards the periphery 

 of the disk, and when that is reached each half becomes 

 distinct. The separation then proceeds down the polypal 

 wall, until ultimately the partition is divided throughout, and 

 two mesenteries, each connected with its own stomoda3um 

 from the beginning, are now fully established, and in all 

 respects resemble the older complete mesenteries. 



The new incomplete mesenteries have a much simpler 

 origin. They are never connected with the stomoda^um, but 

 in serial transverse sections can be seen to arise somewhere 

 towards the upper extremity of the polypal wall, and to extend 

 vertically as well as rad lately for but a short distance. In 

 no case do they pass downwards as far as the incomplete 

 members of the primary series. In one or two instances I 

 have failed to discover them. The incomplete mesenteries 

 never bear filaments, but the complete mesenteries are all 

 filamentiferous for some distance below the inner termination 

 of the stomodffium. The connecting mesenteries, while still 

 unattached to the column wall, also bear filaments at their 

 free edge. 



A knowledge of the early stages of gemmation in ordinary 

 polyps of Mddrepora assists towards an understanding of the 

 peculiar relationships of the mesenteries during the process of 

 iission. Figs. 9-13 represent a series of sections through a 

 very young polyp from near the apex of a branch of M. pro- 

 lifera. The stiige is so early that no tentacles have yet 

 appeared. The bud is formed wholly from the superficial 

 covering of the colony over one of the ordinary peripheral 

 canals, and structurally is entirely unconnected with the 

 older polyjjs, though in communication with them by means 

 of the canal system. I'lie sections are vertical and are taken 

 from only one side of the polyp, but an exactly similar series 

 is found to occur on the other half. 



Fig. 9 is from a section passing through the oral aperture, 

 which has just been formed. The stomodpeal walls appear as 

 simple intumings of tlie superficial wall of the colony. The 

 section is in the directive plane passing through the two 

 directive entocoeles, so that no trace of the mesenteries is 

 included. The two slight elevations of the outer wall are 

 probably the first indications of the axial entocoelic tentacles. 

 The endodermal epithelium of the bud, including that forming 



