26o 



Supplement to ''Nature^' August i8, 1923 



group of embryonic (('lis rta'uUiics tin* LTowth of a 

 neighbouring group. 



Our knowledge of the niutlumry l)y wIik li tlic 

 growth of embryonic tissues is controlled and shaped 

 is likely to increase rapidly, for in recent years em- 

 bryologists have copied the methods invented for the 

 study of bacteria, and have succeeded in growing the 

 live tissues of embryos in artificial media. It has 

 been proved time after time that the epithelial cells of 

 an embryo, such as the living cells of renal tubules, if 

 grown apart from other cells, spread outwards in a more 

 or less disorderly manner ; but if connective-tissue cells 

 are added to the culture, then the epithelial cells form 

 orderly ranks, just as they do in the kidney tubules of 

 the embryo.i3 Carrel ^^ found that the juices of em- 

 bryonic tissues contain substances which cause cultures 

 of living cells of any kind to proliferate rapidly and to 

 continue alive for an endless series of generations. 

 Thus it will be seen that the machinery which regulates 

 the behaviour of groups of cells within the body of the 

 embryo is one of the utmost complexity, and yet is of 

 a kind which can be handled and studied by biologists. 

 Nor can we doubt for a moment that the machinery 

 of development and of growth which we find at work 

 in the embryo is also the machinery of adaptation and 

 of evolution. In every phase of the development and 

 evolution of the human hand we see this adaptational 

 machinery at work. 



Behaviour of Young Nerve Cells. 



There is no need to tell even the uninitiated that 

 the brain and nervous system of man comprises many 

 thousands of millions of microscopic units or nerve cells. 

 Each unit of the brain has its appropriate place in a 

 tremendously complex system, and has its special duty 

 in dealing with the tide of messages which flood that 

 system in every hour of conscious and subconscious life. 

 When a child is born all the nerve centres which regulate 

 the complex apparatus of breathing start into instant 

 and effective operation. When the mother's teat is 

 placed within its lips the nerve centres which regulate 

 this intricate series of actions start to work as if they 

 had served an apprenticeship before they appeared in 

 the orderly development of the babe's nervous system. 

 We cannot yet explain satisfactorily the means by 

 which such really marvellous evolutionary results 

 have been reached, such as reflex nerve centres, ready 

 for action at the moment of birth, but at least we can 

 claim to have before us a prospect of giving a rational 

 account of how the various groups of nerve units are 

 assembled so as to give a functional result. 



Our present knowledge of this matter is largely due 



" Ebeling and Fischer, Joum. Experim. Med., 1922, vol. 34, p. 317. 

 See A. H. Drew, British Joum. Experim. Path., 1922, vpj. 3, p. 20. 



to the researches of Dr. Ariens Kappers " of Amsterdani 

 and to investigations made by his pupils. Nerve cell 

 may not remain in the sites at which they are dcvelo|M< : 

 in their younger stages they Imve the power to migral. 

 Dr. Kappers found that a group of embryonic ner\' 

 cells or neuroblasts, which are afterwards to control 

 definite sets of muscles and therefore to Ik' concemtd 

 in carrying out certain actions of the body, migrat. 

 towards the sources of their information. Tho.^' 

 young executive nerve cells take up their i>ermaneii 

 stations at points most suitable for the performan< ■ 

 of their life's work. If we conceive a mob of war 

 seasoned men to deploy automatically and to take uj 

 effective battle-stations we have before us a piclur« 

 of what is to be seen taking place among the ner\< 

 cells in the brain of the growing human embryc 

 Developing nerve cells send out processes which effect 

 unerring contacts with other distant cell-groups of 

 the body. Dr. Davidson Black ^' found that certain 

 cell - groups on the cortex of the brain proceeded in 

 their development only if the processes of another 

 distant group of cells had entered into contact witli 

 them. We have here another instance of one embryo 

 logical group of cells determining or controlling thi 

 development of another group. Enough has been said 

 to show that the machinery which regulates thi 

 development and growth of the brain is one of th^ 

 utmost complexity. We have no reason to suppose 

 that it is of a kind which lies beyond the comprehension 

 of the human mind, although it may take centuries of 

 neurological inquiry to lay bare its nature. Tin 

 point we are certain of is that the factors which regulait 

 the development, growth, and arrangement of tlu 

 countless units of our nervous system do work in such 

 a way as to produce an effective functional result. 



The Evolution of Muscular Adaptations. 



In no system of the human tody do we find more 

 instructive examples of mechanical adaptation than 

 in the muscles which carry out the movements of our 

 bodies and of our hmbs. The nature of the machinery 

 involved in the elaboration of muscular adaptations 

 may be illustrated by the development of muscles 

 which guard the mouth, eye, nose, and ear, and arc 

 concerned in expression. The bud which gives risc 

 to the muscles on one half of the face begins at one 

 localised site of the human embryo, a site in the 

 embryonic neck, marked by the hyoid arch. From 

 this site the young muscle cells or myoblasts migrate 

 outwards, over the neck and scalp, round the ear, eye, 

 nose, and mouth ; as they reach their destinations 

 they fall into ranks and take up such positions as permit 



" " Further Contributions to Neurobiotaxis," Psyckiat. en Neurolog. 

 Bladen, 1916, Nos. 5, 6. 



" Joum. of Comp. Neur., 1913, vol. 23, p. 351. 



