THE PRINCIPLES OF EMBRYOLOGY 4v7t 
a neural syncytium, which we may call the host, which carries with 
it in its meshes a number of free cells not connected with the nervous 
system. If, then, we confine our attention to the host and trace back 
this neural syncytium to its beginnings in the embryo, we see that, 
from the very nature of the neuro-epithelial couple, each epithelial 
moiety must approach nearer and nearer to its neural moiety, until 
at last it merges with it; the original neuro-epithelial cell results, 
and we must obtain, as far as the host is concerned, a single-layered 
blastula as the origin of all Metazoa. It follows, further, that there 
must always be continuity of growth in the formation of the host, 
i.e. in the formation of the neuro-epithelial syncytium ; that there- 
fore cells which have been previously free cannot settle down and 
take part in its formation, as, for instance, in the case of the formation 
of any part of the gut-epithelium or of muscle-cells from free-living 
cells. 
Further, since the neural moiety is the one element common to 
all the different factors which constitute the host, it follows that the 
convergence of each epithelial moiety to the neural moiety, as we 
pass from the adult to the embryo, is a convergence of all outlying 
parts to the neural moiety, 2.2. to the central nervous system, if there 
is a concentrated nervous system. Conversely, in the commencing 
embryo the place from which the spreading out of cells takes place, 
i.e. from which growth proceeds, must be the position of the central 
nervous system, if the nervous system is concentrated, If the nervous 
system is diffuse, and forms a general sub-epithelial layer, then the 
growth of the embryo would take place over the whole surface of 
the blastula. 
Turning now to the consideration of the second group of tissues, 
those that are not connected with the central nervous system, we 
find that they include among them such special cells as the germinal 
cells, free cells of markedly phagocytic nature, and cells which were 
originally free and phagocytic, but have settled down to form a 
supporting framework of connective tissue, and are known as plasma- 
cells. In the embryo we find also in many cases free cells in the 
yolk, forming more or less of a layer, which function as phagocytes 
and prepare the pabulum for the fixed cells of the growing embryo; 
these cells are known by the name of vitellophags, and in meroblastic 
vertebrate eggs form somewhat of a layer known by the name of 
periblast. Such cells must be included in the second group, and, 
