AND PHYSIOLOGY OF THE BRITISH ASSOCIATION. 705 



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

 gave the name of gastraea 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 stimu- 

 lating and directing embryological research. It is, moreover, 

 very doubtful whether there are to be found in the develop- 

 mental histories of most animals any traces of this gastraea 

 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 hypoblast is the primitive epithelial 

 wall of the alimentary tract. 



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

 remains through life composed of the two layers, which Huxley 

 recognized 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 



O 



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 epider- 

 mis of the simple sack-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 organs so derived have been formed as differentiations of 



