PKRIPHERAL NERVES. 131 



the ventro-lateral corners of the bodjs close to tlie outer border 

 of the nerve-cord, and in the nerve-cords themselves as the 

 •white matter; and I have no doubt that it is present at a still 

 earlier stage, though masked by the large amount of nuclei 

 present. In fact, it may be said of this tissue generally, that 

 it does not become a marked feature of the sections until the 

 organs separate from one another and leave room for the pre- 

 viously closely-packed nuclei to spread out, and, as in the case 

 of the -white matter of the nerve-cords, partly to withdraw them- 

 selves from it (cf. PI. X, fig. 5, and PI. VIII, fig. 39). In 

 whatever manner this tissue may be developed, I think there can 

 be little doubt that it is from its first appearance a continuous 

 tissue, that is to say, the circular fibres at circ. muse, in PI. 

 X, fig. 5, are continuous with the network at neuro-musc, 

 which, in its turn, is continuous with the bundle of fibres forming 

 the nerve, and so with the fibrous matter of the nerve-cords. It 

 thus appears, so far as I have been able to observe the develp- 

 tnent, that the nerves are not formed as outgrowths from the 

 central nervous system, but are parts of a network which 

 originally existed- when the nerve-cords were part of the 

 surface ectoderm. In Stage g, the network is clearly con- 

 tinuous with the surface ectoderm (PI. X, fig. 5). With regard 

 to the commissures connecting the ventral nerve-cords, it 

 seems to me that they also are differentiated in situ from the 

 median ventral ectoderm at a time when the nerve-coi'ds were 

 still parts of the surface ectoderm. I have already said that I 

 do not know the manner in which this network develops ; part 

 of it is undoubtedly formed around the ectodermal nuclei, e.g. 

 the white matter of the cords, the commissures between the 

 cords ; some of it, on the other hand, has, from the first, a 

 relation to the mesodermal nuclei, e. g. the circular fibres at 

 circ. muse, and the network on the ventral side of the feet at 

 PI. X, fig. 5, iieuro-musc. The nerves, therefore, are to be 

 regarded as special differentiations of a pre-existing network, 

 the origin of which is not known, but which at first pervades 

 and is continuous throughout the whole ectodermal and mesc- 

 dermal tissues of the body. 



