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140 tive fibres (fibrils) of muscle. These longitudinal fibres do not appear to represent the original condition of the secondary deposit, but the latter is structureless at first, and its transformation into fibres takes place subsequently. The change seems, however, to commence at a very early period, and indeed before the cavity is completely filled. The transverse striæ of the muscular fasciculi, which, according to my mode of explanation, are produced by the peculiar form of the primitive fibres, likewise make their appearance before the complete filling up of the cell-cavity, as pl. IV, fig. 3, c, exhibits.

According to the observations of Meyen on the formation of the cells of the liber, after the coalescence of the cells and absorption of the septa, a secondary deposit also takes place upon the common cell-membrane in the same way that we have observed to take place in muscle; but I know of nothing amongst vegetables analogous to a secondary deposit consisting of longitudinal fibres. On the contrary, according to Valentin, such deposits appear to take place in plants universally in spiral lines. The beaded appearance which the primitive muscular fibres here and there present, might perhaps be regarded as the result of this tendency to a spiral formation, the intumescences (beads) being so placed, as to produce the transverse striæ, and the latter may perhaps be spiral and not circular. This is, however, a mere conjecture, and requires further research.

The involuntary muscles, such as do not present the transverse striæ, appear to originate in a manner similar to that just described. They differ, however, from the voluntary or striated muscles, in their fibres being generally shorter than those of the latter; probably, therefore, fewer primary cells arrange themselves together to form a secondary cell, and their fibres are commonly thinner and flat. I found in a human uterus, which contained a mature foetus, some long muscular fibres of the breadth of the common primitive fasciculi of voluntary muscles, which were so flat as scarcely to amount to 0.0010 to 0.0015 of a line in thickness. The involuntary muscles, likewise, have cell-nuclei, proving that the fibres composing them do not correspond to the primitive fibres (fibrils), but to the primitive fasciculi of the voluntary muscles. An opposite view of the matter might be taken