486 THE ORIGIN OF VERTEBRATES 
homologous. the epiblast in all cases 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 against 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 origin 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 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. 
