are also clongated in the direction of the fibre. Some of the nuclei, those for instance which appear to be placed on their edges, may possibly become absorbed at the same time, for they never present that position at a later period. The development of the whole cylinder proceeds simultaneously, its granulous aspect disappearing, and the small granules of the cavity also diminishing in quantity. All the stages of transition from the second form into that first described may be observed. The extension does not appear to take place quite regularly, but may be stronger at particular parts, so that, for a considerable extent, a fibre may be somewhat narrow, and present no nucleus, and then again an intumescence oc- curs in which a nucleus lies.
It now, however, becomes a question how the form of muscular fibre last described is generated, and what its elementary form may be. It presented a cylinder, which is most probably hollow, and may be presumed to be closed at both ends, since the muscular fibres terminate abruptly at the tendons, with a well-defined and bluntly-rounded extremity. Cell-nuclei lie within this cylinder at very small distances from one another. Is the cylinder an elongated cell, in which nuclei are formed as the rudiments of new cells, which, however, are not deve- loped; or are the nuclei the remains of cells, which, by coalescence with one another and absorption of the septa, form the entire fibre or cylinder? Or, in other words, is the fibre generated by a coalescence of cells?
I have not observed the stages of transition m which original cells arranged themselves in a linear series to form a fibre, the recent embryos at my command not being sufficiently young for the purpose. I have, indeed, met with an appearance in the form of muscular fibre last described, which might be regarded as an indication that those fibres are composed of small portions joined together. Their margins were incurvated at different spots, and a line, indicative of a division, ran transversely across the entire thickness of the fibre. I have endeavoured to delineate this appearance in pl. IV, fig. 1, b, but I have not succeeded in representing its true character, and it was not, in itself, conclusive. There are some other argu- ments in favour of the fibre of muscle being composed of separate particles. Many of the muscles of fishes or tadpoles, for
