486 THE ORIGIN OF VERTEBRATES 



homologous, the epiblast in all eases forming the external or skin-layer, the 

 hypoblast the internal or gut-layer. 



Such a theory, therefore, as is advocated in this book, which turns the gut 

 of the arthropod into the neural canal of the vertebrate, and makes a new gut 

 for the vertebrate from the external surface must be wrong, as it flatly 

 contradicts the fundamental germ-layer theory. 



Of recent years grave doubts have been thrown upon the validity of this 

 theory, doubts which have increased in force year by year as more and more 

 facts have been discovered which are not in agreement with the theory. So 

 much is it now discredited that any criticism against my theory, which is based 

 upon it, weighs nothing in the balance ag-ainst the positive evidence of recapitu- 

 lation already stated. If the germ-layer theory is no longer credited, upon 

 what fundamental laws is embryology based ? 



In this chapter I have ventured to suggest a reply to this question, based on 

 the uniformity of the laws of growth throughout the existence of the individual. 



In the adult animal the body is composed of two kinds of tissues, those which 

 are connected with or at all events are under the control of the nervous system, 

 and those which are capable of leading a free life independent of the nervous 

 system. These two kinds of tissues can be traced back from the adult to the 

 embryo, and it is the task of embryology to find out how these two kinds of 

 tissue originate. 



The following out of this line of thought leads to the conception that, 

 throughout the Metazoa. the body is composed of a host which consists of the 

 master-tissues of the body, and takes the form of a neuro-epithelial syncytium, 

 within the meshes of which free living independent organisms or cells live, so 

 to speak, a symbiotic existence. 



The evidence points to the orig'in of all these free cells from germ-cells, and 

 thus leads to the conception that the blastula stage of every embryo represents 

 two kinds of cells, the one which will form the mortal host being the locomotor 

 neuro-epithelial cell, the other the independent immortal symbiotic germ-cell. 

 Such conception leads directly to the conclusion that the blastula stage of every 

 member of the Metazoa is the embryonic representation of a Protozoan ancestor 

 of the Metazoa ; an ancestor, whose nature may be illustrated by such a living 

 form as Volvox globator, which, like a blastula. is composed of a layer of cells 

 forming a hollow sphere. These cells partly bear cilia, and so form a locomotor 

 host, partly are of a different character, and form male and female germ-cells. 

 The latter leave the surface of the sphere, pass as free individuals into its 

 fluid contents, form spermaries and ovaries, and then by the rupture of the 

 mortal locomotor host pass out into the external medium, as free swimming 

 young* Volvox. 



It is of interest to note that such members of the Protozoa are among - the 

 most highly developed of the members of this great group. 



From such a beginning 1 arose in orderly evolution, on the one hand, all the 

 neuro-muscular and neuro-epithelial structures of the body — the so-called master- 

 tissues ; on the other, the germ-cells, the blood-corpuscles, lymph-corpuscles 

 plasma and excretory cells, connective tissue cells, cartilage and bone-cells, etc., 

 all of them independent of the central nervous system, all traceable to a 

 modification of the original germ-cells. 



