812 
It is due to his suggestion that I have tested very carefully this point 
by the experimental method. 
The body therefore must elongate posteriorly. This may happen 
by apposition of the material of the germ-ring as is formulated in 
the famous concrescence theory or be due to an elongation of that 
broad triangular thickening of material that early forms and sub- 
sequently narrows as the embryo elongates. A decision may be reached 
by cutting the germ-ring to one or both sides of the embryo. If then 
the embryo elongates normally such growth must be due to the elon- 
gation of the mass independently of the germ-ring. This experiment 
if successful ought to be decisive. 
Two difficulties are met at the start. The germ-ring cannot be 
successfully cut at both sides of the embryo. The germ-ring can easily 
be cut on one side of the embryo but in most such cases the region 
of injury heals rapidly leaving no trace of the cut ends. 
When a hot needle is used the embryo is generally killed. De- 
spite these mechanical difficulties the experiment has been performed 
successfully. In a few cases in which the germ-ring has been cut at 
one side of the embryo with a hot needle the embryo continued to 
develop, although the germ-ring remained attached bat to one side ongly. 
In several cases out of a very large number, where the germ-ring 
was cut in two by a sharp, cold needle the cut ends drew apart and 
did not unite again. From both of these sources the same results 
have been reached. 
The embryo cut off from all connection with the 
germ-ring on one side elongates backwards producing 
an embryo having both right and left sides alike and 
equal. The conclusion follows: In the elongation of 
the embryonic knob backwards the head remains a fixed 
point and the elongation is due to an extension back- 
wards of the mass; the germ-ring takes no important 
part inthe formation of the body of the fish embryo. 
The concrescence theory in its original form as advanced by His 
and in its modified form as advanced by later authors falls to the 
ground if the evidence furnished by their experiments be accepted. 
The theory of concrescence by apposition has if not in its original 
sense yet in a slightly modified form received strong support of late 
by its acceptance by no less authorities than Roux and O. Herrwıc. 
Roux bases his conclusions on his experiments on frogs-eggs. His 
interpretation may be wrong as I shall attempt to show. 
Herrwic finds himself driven to the conclusion by the evidence 
