MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 497 



simplified by regarding the hitter as stomodseal budding; but, as already stated, the step 

 involves an important morphological distinction. Although the stages in division of the mouth 

 or stomodseum have not been actually observed, the results to he detailed below prove conclusively 

 that in fission the stomodseal wall is actually divided into two equal or unequal parts, and that 

 the complete mesenteries inserted on each part go along with it, and help to form the new or 

 daughter polyp. The plane of fission is entocoelic, and usually at right angles to (he directive 

 plane and longer oral axis; hence, only one pair of directives is retained by each of the two 

 primary daughter polyps (p. 505). Were fission to proceed no further in all probability the 

 mesenteries in their later growth would assume the hexameral plan, and the polyps would only 

 differ from a larval or hud polyp in having hut one pair of directives. In most instances, 

 however, the daughter fission polyps are again subjected to fission, so that they never attain a 

 truly regular cyclic character. 



The process of polypal gemmation and fissiparity, as revealed by individual species, will 

 be briefly described. Madrepora, Solenastrsea, and Cladocora will serve as examples of the 

 former, and Manicina and Famia as illustrations of the latter phenomenon. 



BUDDING IX MADREPORA. 



Madrepora is a favorable form on which to study extratentacular gemmation, in a region in 

 which there is no perithecal continuation of the mesenteries (coenosarc). The early stages are 

 reproduced on PI. Ill, figs. 22-27, taken from longitudinal sections of a very young bud, a little 

 below the apex of a branch. 



All the sections represented are from the left side of the median axis of the bud, but the 

 sections on the other side exhibit the same details. The right end is upper in relation to the axis 

 of the branch on which the bud was situated, and the left end is lower. Fig. 22 is from the median 

 dorso-ventral plane passing through the stomodieum and the axial entoeceles. The polyp is yet 

 scarcely raised above the general surface of the ccenosarc; the ridges above and below (right 

 and left in figure) probably indicate the commencement of the axial entocoelic tentatcles, and the 

 included depression the central part of the oral disk. Compared with that of the colony 

 generally, no histological difference is yet presented by the outer ectoderm. Communication 

 between the exterior and the superficial canal system has just been definitely established, the 

 mesogloea of the ccenosarc passing directly into that of the stomoda^al wall. The stomodseal 

 walls bang freely within a superficial longitudinal canal, differing in no important respect from the 

 others around; but as the sections are taken in a longitudinal direction, the canals appear much 

 longer than in the case of transverse sections (PI. I, figs. 2-6). The endoderm of the canal has 

 undergone a marked alteration from that lining the canals and gastric cavity of the polyps. It 

 is broader, more strongly ciliated, non-vacuolated, and zooxanthella 3 are practically absent, though 

 present in the surrounding canals; long, narrow, supporting cells, with abundant protoplasmic 

 contents, are the chief constituents. As best shown in tigs. 23 and 2-1, the endoderm of the canal 

 becomes thinner and more normal toward the periphery of the chamber; the inner and outer 

 layers— ectoderm and endoderm — of the stomoda?al walls are histologically alike. 



The stomodffial wall for a few sections beyond that represented in tig. 22 appears as a 

 projection from the superficial wall of the colony, hanging freely within the canal: the periphery 

 of the projection exhibits four vertical mesoglceal strands, connected with a lower transverse 

 strand. Later, as shown in fig. 23. three central cavities appear and separate the ridges into 

 four distinct components. The lower transverse connecting strand is the horizontal continuation 

 of the stomodseal wall, and the vertical strands represent the mesenteries, not yet separated from 

 one another. The stomodtval wall is continued, as it were, along the free edges of the mesenteries, 

 as often happens in adult polyps. In the next section, fig. 24, the upper and lower (right 

 and left) projections have become free, but the two inner are still united by the stomodseal 

 prolongation. 



The two inner mesenteries afterwards, fig. 25, become free, and now the. uppermost of 

 the four is united with the boundary layer of the canal, and in the later sections ceases to exist. 

 Immediately below the uppermost mesentery is a slight projection of the ccenosarcal endoderm 



