480 THE ORIGIN OF VERTEBRATES 
This conception of the predecessors of the Metazoa being com- 
posed of a mortal host, holding within itself the immortal sexual 
products, leads naturally to the idea of the separate development 
of the host from that of the germ-cells ad initio, so that the 
study of the development of the Metazoa means the study of two 
separate constituents of the metazoan individual on the one 
hand, the elaboration of the elements forming the syncytial host, 
on the other, of those derived from the free-living independent germ- 
cells. The elaboration of the host means the differentiation of the 
protoplasm into epithelial, muscular, and nervous elements, by means 
of which the gonads were carried further afield and their nourishment 
as well as that of the host ensured. 
The réle of the nervous sy8tem as the middleman between internal 
and external muscular and epithelial surfaces was, I imagine, initiated 
from the very earliest time. The further evolution of the host con- 
sisted in a greater and greater differentiation and elaboration of this 
neuro-epithelial syncytium, with the result of a steadily increasing 
concentration and departmental centralization of the main factor of 
the syncytium; in other words, it led to the origin and elaboration 
of a central nervous system. In the interstices of this syncytium 
the gonads were placed, and at first, doubtless, the life of the host 
ended when all the germ-cells had been set free. ‘Reproduce 
and die’ was, I imagine, the law of the Metazoa at its earliest 
origin, and throughout the ages, during all the changes of evo- 
lution, the reminiscence of such law still manifests itself even up 
to the highest forms as yet reached. With the differentiation of 
the syncytial host there came also differentiation of the free-living 
gonads, so that only some of them attained to the perfection of 
independent existence, capable of continuing the species; while others 
became subordinate to the first and provided them with pabu- 
lum, manufacturing within themselves yolk-spherules, and thus in 
the shape of yolk-cells ministered to the developing egg-cell. Thus 
arose a germinal epithelium of which only a few of the elements 
passed out of the host as perfect individuals, the remainder being 
utilized for the nutrition of these few. Such yolk-cells of the 
germinal epithelium would still, however, retain their character as 
free cells totally independent of the syncytial host, and, situated as 
they were between the internal and external epithelium, capable of 
amceboid movement, would naturally have their phagocytic action 
