90 ADAM SEDGWICK. 



connection between nerve and striated muscular fibre. Finally, 

 when he comes to study embryology, the importance and dis- 

 tinctness of the cell meets him at every step, from the complete 

 cleavage which he is led to believe is primitive, to the develop- 

 ment of nerves according to the views of His. 



So much for the student in the schools : now for the investi- 

 gator in the laboratory. He studies the ovum and maintains 

 its absolute isolation in the organism ; or he examines epithelial 

 cells and draws them as isolated structures separated by sharp 

 boundary lines ; or he labours to prove the continuity between 

 the nerve and muscle, or between the nerve and secreting cell : 

 so much is he dominated by the idea of separate cells that he 

 considers that the burden of proof rests rather with the man 

 who asserts such continuity than with him who denies it. 

 Or, if he be an embryologist, he will talk of, and figure, the 

 proliferation of cells at the primitive streak ; he will describe 

 the nascent ganglion cell sending a process from the developing 

 spinal cord into the anterior root, and he will figure it ; he 

 will talk of mesenchyme cells, and figure them for the most 

 part separate from one another. 



I take it that this is a not unfair account of the training a 

 zoologist receives at the present day, so far as the cell is con- 

 cerned, and of the ideas which dominate him in his later work. 

 He believes that the cell is the unit of structure, and that it 

 forms the basis of organisation in the Metazoa ; it is the func- 

 tions of the cell and the relations which it enters into with 

 other cells which forms an important subject of current biolo- 

 gical investigation. Who, then, can deny that the cellular 

 theory of development is still a living power in the school of 

 biology ? That it blinds men's eyes to the most patent facts, 

 and obstructs the way of real progress in the knowledge of 

 structure, it will now be ray endeavour to show. For this pur- 

 pose I shall deal on this occasion with the origin and structure 

 of two tissues of the Vertebrate embryo — the so-called mesen- 

 chyme and the system of peripheral nerve-trunks. My results are 

 the product of many years' work, and will, I hope, be published 

 in greater detail and with figures on a future occasion. 



