498 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



surrounding a mesoglieal iixis. which represents a rudimentary mesenterv; and a few sections 

 beyond, another rudimentary mesentery is revealed. Thus the six mesenteries of one side of 

 the adult polyp of ifadri-jyira are accounted for, four large and two small. The uppei' and 

 lowermost of the four complete mesenteries now begin to diminish in size, and in tig. "id 

 have disappeared at th(> place at which the skeletotrophic layer of the canal is united with the 

 ccenosarc; the uppermost of the two rudimentary mesenteries has likewise passed away. 

 There now remain oidy th(> two middle mesenteries of the larger four, and the lower of the 

 rudimentary two, and of the former, the upper is much larger than the lower. The dilference 

 is more marked in tig. 27, where the lower mesenterv is disappearing, again at the union 

 of the skeletotrophic layer with the superficial wsill; the rudimentary mesentery has already 

 disappeared. A little later, the remaining mesentery has united with the skeletotrophic layer, 

 and is soon lost in sections l)eyond. 



The series of sections illustrates, in the clearest manner, that Inids of Madrepora arise 

 along the superficial wall of a simple external canal, independently of any structural connection 

 with the other polyps, though in comnuuiication with them by means of the canal sx'stem. The 

 six pairs of mesenteries are fully established before the tentacles appear, and the septa are as 

 yet whollv unrepresented. The rudimentary stage of the bud as a whole would indicate that 

 very little time elapses between the development of the ditferent pairs of mesenteries, if, 

 indeed, they do not appear simultaneously. Four of the pairs of mesenteries unite early with 

 the stomodffium, pr()))ably originating along with it; the two remaining pairs are independent 

 of the stomodasum tln-oughout. The complete and incomplete nature of the mesenteries thus 

 early indicated is retained in the adult polyp; for a short distance the four larger complete 

 mesenteries are already united with the skeletotrophic tissues, but the others have a free courses 

 from l)eginning to end. 



The musculature at this .stage is too rudimentary to atford any assistance in determining 

 which are the directive mesenteries. But comparing tlie arrangement with that in transverse 

 sections of 2[adrcjiora (PI. 1), there can be no doubt that the four pairs of mesenteries inserted 

 on the stomodajum represent the eight complete mesenteries, which in the adult extend along the 

 stomodivum. and that the two pairs of rudimentary mesenteries correspond with the free pairs. 

 V and VI, of the adult. The outer mesenteries, right and left, will be the directives III and W . 

 and the two inner pairs, I and II, will be the first and second bilateral pairs. The right end of 

 the sections being uppci- or axial, and the left end lower or abaxial. in relation to the colony, it 

 is clear that the axiul-abaxial relations of the complete and incomplete mesenteries are exactly 

 as in an adult polyj). 



Each of the complete mesenteries, on becoming free from the stomodanun. is somewhat 

 clul)-shaped in section, but no sharp distinction yet exists between the tilamental portion 

 and the epithelium along each face, or, indeed, from the endoderm of the outer wall generally. 

 On Fl. II, fig. 20, is represented the mesentery from another bud, at a somewhat later stage, 

 showing the early development of the tilament. The terminal region stains a little more 

 deeplv than the lateral, owing to the greater closeness of the cells, and consequent greater 

 number of nuclei, but the tissue passes into that of the mesenterial endoderm by an insensible 

 gradation. The gland cells and nematoblasts, cluiracteristic of the adult filament, are not yet 

 distinguishable. 



The strong ciliation of the whole endoderm in the bud, in the earliest stages of development, 

 is probably to be associated with the greater need of bringing an extra supply of mitrient fluid 

 to the rapidly growing parts, while as yet the polyp is unal)le to take in food for itself through 

 the oral aperture. It is only later, when direct connnuiucation with the exterior has been 

 established for some time, that the ditlerent polypal functions l)econie more restricted to special 

 regions, and the.se exhibit corresponding histological modifications. 



The subsequent development of the Mndrejxira bud takes place by an upward growth beyond 

 the general .surface of the colony. In radial polyps the lower (sulcular) region grows more 

 rapidly than the upper (sulcar), so that the former comes to lie outside and the latter on the 



