474 THE ORIGIN OF VERTEBRATES 



sexual cells of the adult. Thus, as he says, from the very earliest 

 period a distinction is made between the " tissue-forming " cells (my 

 syncytial host) and the archreocytes. 



We see, then, that the origin of all these free-living cells can be 

 traced back to the very earliest of the Metazoa. Here between the 

 dermal and gastral layers a gelatinous material, the mesoghea is 

 secreted by these layers. This material is non-living, non-cellular. 

 In it live free cells which may either be germ-cells, amcebocytes, 

 or ' collencytes ' (connective tissue cells). If this mesogloea were a 

 fluid secretion, then we should have a tissue of the nature of blood 

 or lymph ; if it were solid, then we should have the foundation of 

 connective tissue, cartilage, and bone. 



From this primitive tissue it is easy to see how the special 

 elements of the vascular, lymphatic, and skeletal tissues gradually 

 arose, the matrix being provided by the cells of the syncytial host 

 and the cellular elements by the archfeocytes. In fact, we have no 

 right to speak of these lowest members of the Metazoa as not being 

 triploblastic, as possessing nothing corresponding to mesoblast, for 

 in these free cells in the mesogloea we have the origin of the 

 mesenchyme of the higher groups. Thus Lankester, talking of 

 mesenchyme, says : " I think we are bound to bring into considera- 

 tion here the existence in many Ccelentera of a tissue resembling 

 the mesenchyme of Ccelomocoela. In Scyphomedusre, in Ctenophora, 

 and in Anthozoa, branched fixed and wandering cells are found 

 in the mesogloea which seem to be the same thing as a good 

 deal of what is distinguished as mesenchyme in GVelomoccela. 

 These appear to be derived from both the primitive layers ; some 

 produce spicules, others fibrous substance, others again seem to be 

 amcebocytes with various functions. It appears to be probable that, 

 though it may be necessary to distinguish other elements in it, the 

 mesenchyme of Ccelomocoela is largely constituted by cells, which 

 are the mother-cells of the skeletotrophic group of tissues, and are 

 destined to form connective tissues, blood-vessels, and blood." 



Thus we see that the earliest Metazoa were composed of a dermal 

 and gastral epithelium, with a sub-epithelial nervous system con- 

 necting the parts together, which formed, as it were, a host, carrying 

 around free living cells of varying function, all of which may be 

 looked on as derived from arclueocytes, i.e. germ-cells. From these 

 the ccelomatous animals arose, and here also we find, according to 



