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126 state that a transition of these cells into elastic tissue was proved by observation. If, however, such be really the process of formation, as, from analogy, we are entitled to suppose, the bodies of these cells must then take a much more important share in the formation of the fibres than those of areolar tissue do, and the formation of the elastic fibres of the aorta holds a middle position between the generation of the horny fibres in the cortical substance of feathers (see p. 86, and pl. II, fig. 13) and the production of fibres in areolar and fibrous tissues. The reticular appearance of elastic tissue loses its singularity, when it is conceived to be generated in the same manner as those horny fibres in the feather, that is, partly by an elongation of the cells, and partly by a splitting of their bodies. The splitting of the elastic fibres is not to be regarded as an isolated phenomenon, since such division undoubtedly occurs in transitional stages in the development of all forms of areolar and fibrous tissue in the foetus. In this respect the elastic tissue seems to remain at a lower stage of development. Purkinje and Raüschel observed a darkish point in the centre of a transverse section of the elastic fibres of the aorta, and a dotted line in the course of the fibres, and thence inferred the existence of a rudimentary canal in their interior. This supposition, which I must confess formerly struck me as being a very bold one, has much more weight now, inasmuch as it is not improbable that all fibres which are formed by the prolongations of cells (even those of areolar tissue) are hollow, at least, that they are not composed throughout of one uniformly solid mass. If, as an observation of Valentin’s seems to indicate, still more minute fibres may be rendered visible by the aid of caustic potash in those of ordinary elastic tissue, I should be inclined to regard them as analogous to the primitive muscular fibres, whose signification, as we shall subsequently see, differs entirely, in a morphological view, from the primitive fibres of areolar tissue.

Whilst the elastic tissue of the aorta taken from a very young foetal pig exhibited in the manner before described the main characteristics of the tissue, namely, its yellowish colour and elasticity, the ligamentum nuchae of a sheep’s foetus, at a much later period of gestation, was but very slightly developed.