712 DR ALEXANDER BRUCE AND DR JAMES W. DAWSON ON 
GraHAM-KeErrR (1910) has emphasised the necessity of selecting suitable material, 
so that one does not become lost amongst the details of observation. He chose the 
Lepidosiren on account of the coarseness of its histological structure and the size of the 
cell elements. He has come to the following conclusions: that the motor nerve trunk 
is already present as a protoplasmic bridge, placing spinal cord and myotome in organic 
continuity, at a period so early that these structures are in immediate contact, thus 
placing His’s outgrowth theory out of account; that this protoplasmic nerve trunk, at 
first merely granular, gradually assumes a fibrillated structure; that the at-first naked 
and non-nucleated nerve trunk acquires a sheath, the heavily yoked material of whose 
protoplasm demonstrates it to be of mesenchyme origin ; that if the conception of units 
is to be used as a working hypothesis, the unit should be the complex consisting of 
nerve cell, nerve fibre, and muscle cell—a myo-neurone; and that all the possibilities 
seem to point to the nervous system having become evolved out of a sub-epithelial plexus 
of the type which still persists in Coelenterates. The facts of development in Lepidosiren 
thus give strong support to the protoplasmic bridge theory. 
GraHAaM-Kerr looks upon the differentiation of the neuro-fibrils from the physio- 
logical standpoint and regards their specialisation in structure to be correlated to the 
repeated passage of impulses along them. Each particular impulse as it is repeated 
between a central cell and its end cell beats out, as it were, its own special pathway. 
This we term a neuro-fibril. Referring to the recent work of Harrison, he asks if these 
experiments have really the finality which is claimed for them, and suggests the question : 
Has Harrison excluded the possibility that the excised fragments of the embryonic 
spinal cord included the nerve trunk rudiments? He thinks that they simply prove 
that the young nerve grows in length—a self-evident fact quite independent of any 
particular theory. 
Nore oN THE GENESIS OF NERVE FIBRES IN THE CENTRAL NERVOUS SYSTEM, 
Most of the observations upon the origin, development, and structure of nerve fibres 
have been made upon the peripheral nerves. The fibres of the central nervous system 
are described as having no cells either as sheath cells or in any way related to their 
course. Up till recently the theory that each nerve cell and fibre in the central nervous 
system was developed from a single unit was generally accepted. Many writers have 
indeed asserted that it is inconceivable that the fibres of the central nervous system can 
have any cells in relation to them except the central cell of origin. This has been one 
of the strongest arguments of the centralists, that the peripheral nerve fibre also had 
arisen solely from a central cell. FRaGnrto, CapoBIANcHO, and others, however, have 
recently brought forward evidence in favour of the multicellular origin both of the nerve 
cell and central nerve fibre. 
CaPoBIANCHO (1904), in kitten embryos and in the human foetus at the third month, 
has described successive stages in the development of nerve cells from the neuroblasts 
