516 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



The different polyps studied exhibit one or another of three successive stages toward 

 complete fission. These are diagrammatically represented in figs. 18 {cu-c). The primary mesen- 

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

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



Fig. 186. 



Hwlrepora. — Fission continued. The same number of mesenteries are still present within the ventral directive entocoele, and a single 



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



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

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

 has been found. 



Special interest attaches to the mesenteries in figs. 18a, 18b, which connect the two stomoda a al 



Fig. ISc. 

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

 are wholly disconnected. The maerocnemes and mieroenemes 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, i <■/, pi. I, fig. 4.) 



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

 has ceased altogether. In the paper already mentioned, it has been shown that these connecting 

 mesenteries are a result of the practically simultaneous division of the primary stomocheuin and 

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



