498 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



surrounding a mesoglceal axis, which represents a rudimentary mesentery; and a few sections 

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

 the adult polyp of Madrepora are accounted for, four large and two small. The upper and 

 lowermost of the four complete mesenteries now T begin to diminish in size, and in tig. 26 

 have disappeared at the 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 only the 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 difference 

 is more marked in fig. 27, where the lower mesentery is disappearing, "again at the union 

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

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

 and is soon lost in sections beyond. 



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

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

 with the other polyps, though in communication with them by means of the canal system. The 

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

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

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

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

 the stomodseum, probably originating along with it; the two remaining pairs are independent 

 of the stomodseum throughout. 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 course 

 from beginning to end. 



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

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

 sections of Madrepora (PI. I), there can be no doubt that the four pairs of mesenteries inserted 

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

 stomodseum, 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 IV, 

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

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

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

 as in an adult polyp. 



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

 club-shaped in section, but no sharp distinction yet exists between the nlamental portion 

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

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

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

 deeply 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, characteristic 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 nutrient fluid 

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

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

 established for some time, that the different polypal functions become more restricted to special 

 regions, and these exhibit corresponding histological modifications. 



The subsequent development of the Madrepora 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 conies to lie outside and the latter on the 



