112 SECTIONAL ADDRESSES, 
method of inheritance in the examples chosen for his experiments, for 
in 1902 Sutton pointed out that an application of the facts then known 
as to the behaviour of the chromosomes would provide an explanation 
of the observed facts of Mendelian inheritance. In the same year 
McClung suggested that the accessory chromosome in the male germ- 
cells is a sex-determinant. These two papers may be taken as the 
starting-point of that vast series of researches which have gone far 
toward the elucidation of two of the great problems of biology—the 
structural basis of heredity and the nuclear mechanism correlated with 
sex. The evidence put forward by Morgan and his colleagues, resulting 
from their work on Drosophila, would seem to permit little possibility 
of doubt that factors or genes are carried in the chromosomes of the 
gametes, and that the behaviour of the chromosomes during maturation 
of the germ-cells and in fertilisation offers a valid explanation of the 
mode of inheritance of characters. The solution of this great riddle of 
biology has been arrived at through persistent observation and experi- 
ment and by critical analysis of the results from the point of view of 
the morphologist, the systematist, the cytologist, and the geneticist. 
Among other important developments in the period, reference may 
be made to the great activity in investigation of the finer structure of 
the nerve-cell and its processes. By 1891 the general anatomical rela- 
tions of nerve-cells and nerve-fibres had been cleared up largely through 
the brilliant work of Golgi and Cajal on the brain and spinal cord, and 
of von Lenhossék, Retzius, and others on the nervous system of annelids 
and other invertebrates. In these latter had been recognised the receptor 
cells, the motor or effector cells, and intermediary or internunciary 
cells interpolated betw2en the receptors and effectors. In June 1891 
Waldeyer put forward the neurone theory, the essence of which is that 
the nerve-cells are independent and that the processes of one cell, though 
coming into contiguous relation and interlacing with those of another 
cell, do not pass over into continuity. He founded his views partly 
upon evidence from embryological researches by His, but chiefly on 
results obtained from Golgi preparations and from anatomical investiga- 
tions by Cajal. The neurone theory aroused sharp controversy, and 
this stimulus turned many acute observers—zoologists and histologists— 
to the intimate study of the nerve-cell. First among the able opponents 
of the theory was Apathy, whose well-known paper, published in 1897, 
on the conducting element of the nervous system and its topographical 
relations to the cells, first made known to us the presence of the neuro- 
fibrillar network in the body of the nerve-cell and the neurofibrils in the 
cell-processes. Apathy held that the neurofibrillar system formed a 
continuous network in the central nervous system, and he propounded 
a new theory of the constitution of the latter, and was supported in 
his opposition to the neurone theory by Bethe. Nissl, and others. The 
controversy swung to and fro for some years, but the neurone theory— 
with certain modifications—seems now to have established itself as a 
working doctrine. The theory first enunciated as the result of morpho- 
logical studies receives support from the experimental proof of a slight 
arrest of the nerve-impulse at the synapse between two neurones, which 
causes a measurable delay in the transmission. The latest development 
