THE PRINCIPLES OF EMBRYOLOGY 483 
therefore be called hypoblast, spreads continuously over the yolk, as 
also does the surface epithelial layer, or epiblast. Such spreading 
is always a continuous one for both surfaces, so that the yolk is 
gradually enclosed by a continuous orderly growth from the germ- 
band, and not by the settling down of free cells in the yolk here 
and there to form the gut-lining. This steady orderly development 
proceeds owing to the nourishment afforded by the activity of the 
free cells or vitellophags and the absorbing power of the hypoblast, 
a steady growth round the yolk which results in the formation of the 
gut-tube, the outer covering and all the muscular aud excretory 
organs. Where, then, is this starting-point, this germ-band from 
which the whole embryo grows? It forms the mid ventral area of 
the adult animal, it corresponds exactly to the position of the 
central nervous system. The whole phenomenon of embryonic 
growth in the scorpion is exactly what must take place on the 
argument. deduced’ from the study of the adult that the animal 
arises as a neuro-epithelial syncytium, and we see that that layer of 
cells which is situated next to the food-material forms the alimen-. 
tary tube. It is not a question whether such layer is ventral or 
dorsal to the neural cells, but whether it is contiguous to or removed 
from the food-material. 
Take, again, a meroblastic vertebrate egg as of the bird. Again we 
find free cells passing into the yolk to act as vitellophags, the so-called 
periblast cells; again we see that the embryo starts from a germ- 
band or embryonic shield, and spreads from there continuously and 
steadily ; again we see that the layer of cells which lies against the 
yolk absorbs the fluid pabulum for the growing cells; again we see 
that the area from which the whole process of growth starts is that 
of the central nervous system, and again we see that those cells 
which are contiguous to the food form the commencing gut, and are 
therefore called hypoblast, though in this case they are ventral not 
dorsal to the neural layer. 
The comparison of these two processes shows that there is one 
common factor, one thing comparable in the two, one thing that is 
homologous and is the essential in the formation of that part of the 
animal which I have called the host, and that is the central nervous 
system. Whether the epithelial layer which lies ventrally to it or the 
one that is dorsal forms the gut depends upon the position of the 
food-mass. Where the food is, there will be the absorbing layer. 
