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 ab 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 role of the nervous system 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. ' Eeproduce 

 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 

 amoeboid movement, would naturally have their phagocytic action 



