November 17, 1923] 



NA TURE 



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|)aralleled by increase in our knowledge of the germ- 

 rells, especially during their maturation into eggs and 

 > perms, the utmost refinements of technique and 

 ' hservation having been brought to bear on these and 

 n other cells. During the last thirty years, and 

 ^ >pecially during the latter half of this period, cytology 

 has developed so rapidly that it has become one of the 

 lost important branches of modern biology. One 

 : the landmarks in its progress was the appearance. 

 at the end of 1896, of E. B. Wilson's book on " The 

 'ell." A great stimulus to cytological work resulted 

 ')m the rediscovery in 1900 of the principle of heredity 

 iblished by Mendel in 1865, which showed that a 

 latively simple conception was sufficient to explain 

 t* method of inheritance in the examples chosen for 

 s experiments, for in 1902 Sutton pointed out that 

 1 application of the facts then known as to the be- 

 iviour of the chromosomes would provide an explana- 

 in of the observed facts of Mendelian inheritance. 

 1 the same year McClung suggested that the accessory 

 romosome in the male germ-cells is a sex-determinant. 

 lese two papers may be taken as the starting-point 

 that vast series of researches which have gone far 

 ward the elucidation of two of the great problems of 

 ology — the structural basis of heredity and the nuclear 

 chanism correlated with sex. The evidence put 

 ward by Morgan and his colleagues, resulting from 

 :.;cir 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 ex- 

 planation 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 investi- 

 gating the finer structure of the nerve-cell and its 

 processes. By 1891 the general anatomical relations 

 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 Lenhossek, 

 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 intemunciary cells inter- 

 polated between the receptors and effectors. In June 

 1891 Waldeyer put forward the neurone theory, the 

 essence of which is that the nerve-ccUs arc 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 embryo- 

 logical 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 ol)servers — 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 ^\<^i-m 



NO. 2820, VOL. I 12] 



and its topographical relations to the cells, first made 

 known to us the presence of the neurofibrillar 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 theor\- — with certain 

 modifications — seems now to have established itself as 

 a working doctrine. The theory first enunciated as 

 the result of morphological 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 in morphological work on 

 nerve-elements is the investigation of the neuromotor 

 system in the Protozoa. Sharp (1914), Yocom (1918), 

 and Taylor (1920), working in Kofoid's laboratory, 

 have examined this mechanism in the ciliates Diplo- 

 dinium and Euplotes, and they describe and figure a 

 mass — the neuromotorium — from which fibrils pass to 

 the motor organs, to the sensory lip, and, in Diplo- 

 dinium, to a ring round the oesophagus. The function 

 of the apparatus is apparently not supporting or 

 contractile, but conducting. By the application of 

 the finest methods of microdissection, specimens of 

 Euplotes have been operated upon while they were 

 observed under an oil-immersion objective. Severance 

 of the fibres destroyed co-ordination between the 

 membranelles and the cirri, but other incisions of 

 similar extent made without injuring the fibrillar 

 apparatus did not impair co-ordination, and experi- 

 ments on Paramsecium by Rees (1922) have yielded 

 similar results. While the experimental evidence is 

 as yet less conclusive than the morphological, it 

 supports the latter in the view that the fibrils have 

 a conducting, co-ordinating function. Progress in our 

 knowledge of the nervous system is but one of many 

 lines of advance in our understanding of the correlation 

 and regulation of the component parts of the animal 

 organism. 



The ciliate Protozoa have been the subject during 

 the last twenty years of a series of investigations of 

 great interest, conducted with the purpose of ascertain- 

 ing whether decline and death depend on inherent 

 factors or on external conditions. While these re- 

 searches have been in progress we have come to realise 

 more fully that ciliates are by no means simple cells, 

 and that some of them are organisms of highly complex 

 structure. Twenty years ago Calkins succeeded in 

 maintaining a strain of Paramecium for twenty-three 

 months, during which there were 742 successive 

 divisions or generations, but the strain, which had 

 exhibited signs of depression at intervals of about 

 three months, finally died out, apparently from 

 exhaustion. From this work, and the previous work 

 of Maupas and Hcrtwig, the opinion became general 

 that ciliates are able to pass through only a limited 

 number of divisions, after which the animals weaken, 

 become abnormal and die, and it was lielievcd that 

 the only way l)y which death could be averted was by 

 a process of mating or conjugation involving an inter- 

 change of nuclear materia 1 between the two conjugants 



