cea: 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 archzocytes. 
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 mesoglcea 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 mesoglea 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 archeocytes. 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 mesoglcea 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 Celentera of a tissue resembling 
the mesenchyme of Ceelomocela. In Scyphomedusce, in Ctenophora, 
and in Anthozoa, branched fixed and wandering cells are found 
in the mesoglcea which seem to be the same thing as a good 
deal of what is distinguished as mesenchyme in Ccelomoceela. 
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 Ccelomocela 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 archzocytes, 7.c. germ-cells. From these 
the ccelomatous animals arose, and here also we find, according to 
