4i8 



NATURE 



\_Scpt. 2, iSSo 



the same species. If th«ef are it can be shown that the embryo'' 

 of existing forms pass through stages in which they have the 

 characters of more primitive groups, a sufficient proof of our 

 proposition will have been given. 



That such is crten the case is a well known fact, and was even 

 known before the publication of Darwin's works. Von Ener, 

 I lie greatest embryologist of the century, who died at an advanced 

 aje but a few years ago, discussed the proposition at consider- 

 .nble length in a work published between the years J830 and 

 1S40. He came to the conclusion that the embryos of higher 

 forms never actually resemble lower forms, but only the em- 

 bryos of lower forms ; and he further maintained that such 

 resemblances did not hold at all, or only to a very small extent, 

 beyond the limits of the larger groups. Thus he believed that, 

 though the embryos of Vertebrates might agree amongst them- 

 selves, there \\as no resemblance between them and the embryos 

 of any invertebrate group. We now know that these limitations 

 of vonBaer do not hold good, but it is to be remembered that 

 the meaning now attached by embryologists to such resemblances 

 was quite unknown to him. 



These preliminary remarks will, I trust, be sufficient to de- 

 monstiiate how completely modern embi-yological reasoning is 

 dependent on the two laws of inheritance and variation, which 

 constitute the keystones of the Darwinian theory. 



Before the appearance of the "Origin of Species" many very 

 valuable embryological investigations were made, but the facts 

 discovered were to their authors merely so many ultimate facts, 

 which admitted of being classified, but could not be explained. No 

 explanation could be offered of why it is that animals, instead of 

 developing in a simple and straiglitforward way, undergo in the 

 course of their growth a series of complicated changes, during 

 which they often acquire organs which have no function, and 

 which, after remaining visible for a short time, disappear without 

 leaving a trace. 



No explanation, for instance, could be offered of why it is that 

 a frog in the course of its growth has a stage in which it breathes 

 like a fish, and then why it is like a newt with a long tail, which 

 gradually becomes absorbed, and finally disappears To the 

 Darwinian the explanation of such facts is obvious. The^tage 

 when the tadpole breathes by gills is a repetition of the stage 

 when the ancestors of the frog had not advanced in the scale of 

 development beyond a fish, while the newt-like stage implies 

 that the ancestors of the frog were at one time organised very 

 much like the nents of to-day. The explanation of such facts 

 has opened out to the embryologist quite a new series of pro- 

 lilems. These problems may be divided into two main groups, 

 technically known as those of phylogeny and those of organo- 

 geny. The problems of phylogeny deal with the genealogy of 

 the animal kingdom. A complete genealogy would form what 

 is known as a natural classification. To attempt to form such a 

 classification has long been the aim of a large number of natu- 

 r.alists, and it has frequently been attempted without the aid of 

 embryology. The statements made in the earlier part of my 

 address clearly show how great an assistance embryology is 

 capable of giving in phylogeny; and as a matter of fact embry- 

 ology has been during thj last few years very widely employed 

 in all phylogenetic questions, and the results which have been 

 arrived at have in many cases been very striking. To deal with 

 these results in detail would lead me into too technical a depart- 

 ment of my subject ; but I may point out that amongst the more 

 striking of the results obtained ir«/f;Y/j' by embryological methods 

 is the demonstration 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 are now universally placed with the Vertebrata. 



'I he 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 devot- 

 ing 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 techni- 

 cally 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 

 lowdy-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 divi- 

 sion 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 con- 

 stitute 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 secretoiy 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 wdiich takes place con- 

 sists 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 centuiy 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 1S49, and at greater length in 1859, Huxley demonstrated 

 that the bodies of all the polype tribe or Ccclenterata — that is to 

 say, of the group to which the common polype, jelly-fish, and 

 the sea-anemone belong — were composed 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 i860 a 

 young Russian naturalist named Kowalevsky began to study by 

 special histological methods the development of a number of 

 invertebrated 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 importance of these discoveries, and 

 they formed the basis of two remarkable essays, one by our ow'n 

 countryman, Prof. Lankester, and the other by a distinguished 

 German naturalist. Prof. Haeckel of Jena. 



In these essays the attempt was made to show that ■ the 

 stage in development already.spoken of, in which the cells are 

 arranged in the form of tw o layers inclosing a central cavity, has 

 an ancestral meaning, and that it is to be interpreted to signify 

 that all the Metazoa are descended from an ancestor which had 

 a more or less oval form, with a central digestive cavity provided 

 with a single opening, seri'ing both for the introduction of food 

 and for the ejection of indigestible substances. The body of 

 this ancestor was supposed to have been a double-walled sac 

 formed of an inner layer, the hypoblast, lining the digestive 

 cavity, and an outer layer, the epiblast. To this form Haeckel 

 gave the name of gastraea or ga^trula. 



There is every reason to think that Lankester and Haeckel were 

 quite justified in concluding that a form more or less like that 

 just described was the anctstor of the Metazoa ; but the further 

 speculations contained in their essays as to the origin of this 

 form from the Protozoa can only be regarded as suggestive 

 feelers, which, however, have been of great importance in 

 stimulating and directing embryological research. It is, more- 

 over, very doubtful w liether there are to be found in the develop- 

 mental histories of most animals any traces of this gastrrea 

 ancestor, other than the fact of their passing through a stage in 

 which the cells are divided into two germinal layers. 



The key to the nature of the two germinal layers is to be 

 found in Huxley's comparison between them, and the two layers 

 in the fresh-water polype and the sea-anemone. The epiblast 

 i^ the primitive skin, and the hypoblast is the primitive epithelial 

 wall of the alimentary tract. 



In the whole of the polype group, or Crelenterata, the body 

 remains through life composed of the two layers, which Huxley 

 recognised as homologous with the epiblast and hypoblast of the 

 Vertebrata ; but in all the higher Metazoa a third germinal 



