640 EEPORT— 1880. 



In these essaj^s the attempt was made to show that the stape in development 

 already spoken of, in which the cells are arranged in the form of two layers en- 

 closing a central cavity has an ancestral meaning, and that it is to he 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, 

 serving 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 hA-poblast, lining the digestive cavity, and an 

 outer layer, the epiblast. To this form Haeckel gave the name of gastrsea or 

 gastrula. 



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 ancestor 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, moreover, very doubtful whether there are to be found in the de- 

 velopmental histories of most animals any traces of this gastrsea 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 is the primitive skin, and the lij-poblast is the primitive 

 epithelial wall of the alimentary tract. 



In the whole of the pohpe group, or Ocelenterata, 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 layer, known as the mesoblast, early makes its appearance between the 

 two primary layers. The mesoblast originates as a differentiation of one or of 

 both the primary germinal layers ; but although the different views which have 

 been held as to its mode of origin form an important section of the history of 

 recent embryological investigations, I must for the moment confine myself to 

 saying that from this layer there take their origin — the whole of the muscular 

 system, of the vascular system, and of that connective-tissue system which forms 

 the internal skeleton, tendons, and other parts. 



We have seen that the epiblast represents the skin or epidermis of the simple 

 sac-like ancestor common to all the Metazoa. In all the higher Metazoa it gives 

 rise, as might be expected, to the epidermis, but it gives rise at the same time to a 

 number of other organs ; and, in accordance with the principles laid down in the 

 earlier part of my address, it is to be concluded that the orcjans so derived have been 

 formed as differe^itiations of the primitive epidermis. One of the most interesting of 

 recent embryological discoveries is the fact that the nervous system is, in all but a 

 very few doubtful cases, derived from the epiblast. This fact was made out for 

 vertebrate animals by the great embryologist Von Baer ; and the Russian naturalist 

 liowalevsky, to whose researches I have already alluded, showed that this was 

 true for a large number of invertebrate animals. The derivation of the nervous 

 system from the epiblast has since been made out for a sufficient number of forms 

 satisfactorily to establish the generalisation that it is all but universally derived 

 from the epiblast. 



In any animal in which there is no distinct nervous system, it is obvious that 

 the general surface of the body must be sensitive to the action of its surroundings, 

 or to what are technically called stimuli. We know experimentally that this is so 

 in the case of the Protozoa, and of some very simple Metazoa, such as the fresh- 

 water Polype or Hydra. The skin or epidermis of the ancestor of the Metazoa 

 was no doubt similarly sensitive ; and the fact of the nervous system being 

 derived from the epiblast implies that the functions of the central nervous 

 system, which were originally taken by the whole skin, became gradually 

 concentrated in a special part of the skin which was step by step removed 

 from the surface, and finally became a well-defined organ in the interior of the 

 body. 



