OUTGROWTH OF THE NERVE FIBER 
823 
or prevent fusion between contiguous elements^ and the same 
elements might therefore at one time be separate, and at another 
continuous. 
In some cases large numbers of the protoplasmic fibers have 
been found matted together, in an inextricable tangle (fig. 17). 
These have undoubtedl}^ been derived from groups of cells, and 
the specimen reminds one of the condition that is found after 
the medullary cord of the embryo is removed from the trunk re- 
gion, when the fibers from the brain grow out in a large bundle 
and lose themselves ultimately in the mesenchyme. 
DISCUSSION OF RESULTS 
The significance of the experiments in the interpretation of normal 
development 
In attempting to estimate the significance of the foregoing ex- 
periments as elucidating the processes of normal development, 
we are at once confronted with the question whether the conditions 
in the experiments are sufficiently like those in the embryonic 
body to warrant any comparison at all. This can be answered 
most satisfactorily by carefully comparing the activities of isolated 
tissues with the activities of the tissues in the normal embryo. 
Such an empirical determination must have more weight than any 
amount of a priori argumentation upon the subject. The phe- 
nomena which can be compared and interpreted most readily are 
those of movement and of tissue differentiation. 
The movement of the embryonic cells in the lymph clot is very 
distinct, and is due beyond doubt to the activities of the hyaline 
ectoplasm (figs. 23 and 27), which is accumulated especially at the 
angles of the cells. It there forms extremely fine filamentous 
pseudopodia, through the activity of which the cells may change 
their shape or move from place to place. The exact character of 
the movement is not the same in all kinds of cells and it varies 
greatly in intensity. Axial mesoderm and medullary cord yield 
cells that frequently wander for considerable distances by them- 
selves; epidermis, when it does not roll up into bands or spheres, 
