AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 703 



naturalists, separated by a wide gulf from the Invertebrata, but 

 that there is a group of animals, known as the Ascidians, formerly 

 united with the Invertebrata, which are now universally placed 

 with the Vertebrata. 



The discoveries recently made in organogeny, or the genesis 

 of organs, have been quite as striking, and in many respects 

 even more interesting, than those in phylogeny, and I propose 

 devoting the remainder of my address to a history of results 

 which have been arrived at with reference to the origin of the 

 nervous system. 



To render clear the nature of these results I must say a few 

 words as to the structure of the animal body. The body is 

 always built of certain pieces of protoplasm, which are technically 

 known to biologists as cells. The simplest organisms are com- 

 posed either of a single piece of this kind, or of several similar 

 pieces loosely aggregated together. Each of these pieces or 

 cells is capable of digesting and assimilating food, and of 

 respiring; it can execute movements, and is sensitive to ex- 

 ternal stimuli, and can reproduce itself. All the functions of 

 higher animals can, in fact, be carried on in this single cell. 

 Such lowly organized forms are known to naturalists as the 

 Protozoa. All other animals are also composed of cells, but 

 these cells are no longer complete organisms in themselves. 

 They exhibit a division of labour : some carrying on the work 

 of digestion ; some, which we call nerve-cells, receiving and 

 conducting stimuli ; some, which we call muscle-cells, altering 

 their form in fact, contracting in one direction under the 

 action of the stimuli brought to them by the nerve-cells. In 

 most cases a number of cells with the same function are united 

 together, and thus constitute a tissue. Thus the cells which 

 carry on the work of digestion form a lining membrane to a 

 tube or sack, and constitute a tissue known as a secretory epi- 

 thelium. The whole of the animals with bodies composed of 

 definite tissues of this kind are known as the Metazoa. 



A considerable number of early developmental processes are 

 common to the whole of the Metazoa. 



In the first place every Metazoon commences its existence 

 as a simple cell, in the sense above defined ; this cell is known 

 as the ovum. The first developmental process which takes 



