842 
ROSS GRANVILLE HARRISON 
One characteristic that the embryonic cells have in common, is 
the power of movement. They change their form or move from 
place to place in the clot by virtue of the amoeboid activity of their 
hyaline ectoplasm. The amount of activity and its result vary 
according to the tissue, cells from the nervous system and the 
mesoderm being most active, while those of the endoderm and 
notochord are most inert. 
In the case of cells from the medullary tube and the primordia of 
the cranial ganglia the activity is so localized and the ductility 
of the ectoplasm is such, that the movement results in the forma- 
tion of long fibers, the primitive axones. The free end of each 
fiber is enlarged and provided with fine processes or pseudopodia. 
This part continues its progression and the fiber is gradually drawn 
out. 
The rate of progression (lengthening of the fiber) varies con- 
siderably, the extremes observed being 15.6^ per hour (lOO/x in 
6 hours 25 minutes) and 56m per hour (44/x in 47 minutes). 
The longest fiber observed, and this was followed throughout 
its whole period of growth (53 hours), was 1.15 mm. long. 
The nerve fibers take origin usually from masses of cells 
which are so opaque that their mode of connection with the cells 
cannot be made out, but in a considerable number of cases the 
fibers were seen to originate in single isolated cells, the longest 
one of this kind having had a total length of 631m and having 
had several long branches. 
In many cases anastomoses have been found between fibers. 
These have been observed to form through secondary fusion, but 
two threads oncoming together do not necessarily fuse, and anas- 
tomoses already formed may be resolved later. 
The experiments show that neuroblasts are competent to form 
primitive nerve fibers wdthin a foreign unorganized medium sim- 
ply by the amoeboid outgrowth of their protoplasm. By elim- 
inating from the periphery all formed structures which have here- 
tofore been supposed to transform themselves into nerve fibers 
and leaving only the neuroblasts in the field, it is demonstrated 
that the latter are the sole elements essential to the formation of 
nerves. The concepts of both Hensen and Held are rendered 
untenable. 
