516 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The different polyp.s studied exhibit one or another of three successive stages toward 

 complete fission. These are diag-rammaticalh' represented in tigs. 18 («-c). The primary mesen- 

 teries are indicated by thicker lines, and are numbered from I to VI; the new mesenteries are 

 denoted l)y the letters A to F, no successive order in their appearance being assumed thereby. 



Fig. IKii. 



Madrepora.— Fission continuetl. The same number of mesenteries are still present within the ventral directive entoeoele, and a single 



pair (E, E) occurs within the dorsal directive entoccele, while only one mesenterial strand connects the stomodsea. 



The figures reveal that new mesenteries are added in bilateral pairs, disposed axialh' in both the 

 dorsal and ventral entocceles. No stage in which less than twenty-four mesenteries were present 

 has been found. 



Special interest attaches to the mesenteries in figs. 1S«, ls/>. which connect the two stomodical 



Fig. ISf. 

 Madrepora. — Fission continued. Two mesenterial pairs (E. F) now occur within the dorsal directive entoeoele, and the stomodseal tubes 

 are wholly disconnected. The macrocnemes and microcnemes and arrangement of the musculature on the faces are such that if the 

 polyp were divided into two halves along the median axis the mesenterial arrangement in each polyp would be the same as in an 

 ordinary polyp with only six pairs of mesenteries. {(■/, PI. I, fig. 4.) 



tubes. In fig. 18« two of these are present, in fig. lsl> only one, while in fig. 18c the connection 

 has ceased altogether. In the paper ali-eady mentioned, it has been shown that these connecting 

 mesenteries are a result of the pi-actically simultaneous division of the primary stomodieuin and 

 the appearance of the new mesenteries. The adjacent mesenteries, situated between the two 



