TBANSACTIONS OF SECTION D. — DEPT. ANATOMY AND PHTSIOLOGT. 639 



assistance embryologj' is capable of giving in phylogeny ; and as a matter of fact 

 embryology bas been during the last few years very widely employed in all phylo- 

 genetic questions, and the results which have been arrived at have in manv cases 

 been very striking. To deal with these results in detail would lead me into too 

 technical a department of my subject ; but I may point out that amongst the more 

 striking of the results obtained entirely by embryological methods is the demon- 

 stration that the Vertebrata are not, as was nearly universally believed by older 

 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 is now universally placed in the same class 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 

 structm-e of the animal body. The body is always built of certain pieces of proto- 

 plasm, which are technically known to biologists as cells. The simplest organisms 

 are composed 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 

 external stimuli, and can reproduce itself. All the functions of higher animals can, 

 in fact, be carried on in this single cell. Such lowly organised 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 sac, and 

 constitute a tissue known as a secretory epithelium. 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 place consists in the division or segmentation of the single cell 

 into a number of smaller cells. The cells then arrange themselves into two 

 groups or layers known to embryologists as the primary germinal layers. These 

 two layers are usually placed one within the other round a central cavity. The 

 inner of the two is called the hypoblast, the outer the epiblast. The existence of 

 these two layers in the embryos of vertebrated animals was made out early in the 

 present century by Pander, and his observations were greatly extended by Von Baer 

 and Remak. But it was supposed that these layers were confined to vertebrated 

 animals. In the year 1849, and at greater length in 1859, Huxley demonstrated 

 that the bodies of all the polype tribe or Ocelenterata — that is to say of the group 

 to which the common polype, the jelly-fish, and the sea-anemone belong — were com- 

 posed of two layers of cells, and stated that in his opinion these two layers were 

 homologous with the epiblast and hypoblast of vertebrate embryos. This very 

 brilliant discovery came before its time. It fell upon barren ground, and for a 

 long time bore no fruit. In the year 1866 a young Russian- naturalist named 

 Kowalevsky began to study by special histological methods the development of 

 a number of iuvertebrated forms of animals, and discovered that at an early 

 stage of development the bodies of all these animals were divided into germinal 

 layers like those in vertebrates. Biologists were not long in recognising the im- 

 portance of these discoveries, and they formed the basis of two remarkable essays, 

 one by our own countryman, Professor Lankester, and the other by a distinguished 

 German naturalist, Professor Haeckel, of Jena. 



