76 PATTEN. [Vou. XII. 
of much assistance. I doubt, however, whether the most perfect 
sections would indicate any materially different condition from 
that seen in the surface views. It is somewhat surprising that 
the tension necessary to push the old halves over the yolk does 
not produce in them any other distortion than a gentle curvature 
to right and left. It shows how truly each part assumes its char- 
acteristic forms, dominated by its inherent structure rather than 
by the mechanical stress of adjacent organs. For example, in 
swinging the head of either embryo to the right or left, the 
movement may coincide on one side with certain lines of 
growth, but be directly opposed to them on the other. This 
may be seen in the diagram, Fig. 7, where it is obvious 
that the left side of the right-hand embryo is being swept 
along in the direction of its own lateral growth, and it must 
receive some additional impulse with which to overcome the 
resistance to it. But on the right side of the same embryo 
the movement is against the line of lateral growth. The 
internal stress at the lateral C’s is very different from that 
at corresponding points in a single embryo, and very different 
from what it is at the median C, and yet the resulting organs 
under these diverse conditions are the same! 
The reason these varying mechanical conditions have so 
little effect on the form of the organs is probably because they 
are so transitory. They differ essentially from the permanent 
and gradually increasing stresses that produce concrescence 
and degeneration. 
In Fig. 92 the separation into two embryos began at an 
earlier period and is carried much farther than in the ones just 
considered. The head of each embryo has been swept over 
an arc of 90°. Further movement in that direction is pre- 
vented by the interference of the lower margins (as the figure 
stands) of the old mesodermic area. It is probable that the 
tension produced by this interference would about equal the 
devaricating force in the new halves, as soon as the two 
embryos formed a straight line tail to tail. If this condition is 
reached at an early period, the same forces, 7.e. the tendency 
of the posterior margin of the mesodermic area to concresce 
behind cach embryo, will ultimately force them apart. But 
