OUTGROWTH OF THE NERVE FIBER 
811 
Similar filaments are often found extending from one cell to 
another (figs. 17 and 27). 
The histological differentiation of the tissue in successful prep- 
arations is specific and normal, although it does not proceed so 
rapidly nor become so complete as when the tissues are left in 
their normal environment. No doubt one factor which contrib- 
utes to this retardation of development is the insufficient supply 
of oxygen within the moist chamber. This is indicated by the 
slow rate of absorption of the yolk, as compared with its rate of 
absorption in the embryo. It is only in those specimens that have 
been kept alive for a week or longer that there is any great diminu- 
tion of the yolk contained in the cells. The most noticeable his- 
tological differentiations that have been observed in the various 
isolated tissues are the following: the formation of typically 
striated fibrillar substance in cells taken from the axial mesoderm ; 
the development of the cuticular border in ectoderm cells and the 
growth of cilia which may continue in action for days ; the forma- 
tion of typical chromatophores, most probably from cells derived 
from the medullary cord; and the formation from the central 
nervous system and from the cranial ganglia, of the long proto- 
plasmic filaments, which are identical with the nerve fibers of the 
embryo and which are the especial subject of the present investi- 
gation. 
Muscle fibers have been found not only in cases where a portion 
of the axial mesoderm was left in contact with the medullary 
cord, but also where it was isolated entirely from all other tissue, 
showing that the cells of the muscle plates at this stage, i.e., be- 
fore visible differentiation has begun, have the power of self -differ- 
entiation in the highest possible degree. This is in conformity 
with the results obtained from the study of muscle tissues in em- 
bryos deprived of the central nervous system. It is of course 
only in certain favorable cases that the presence of muscles fibril- 
lae can be observed in the fresh specimen, since the tissue, unless 
it spreads out, is too opaque to permit of satisfactory observation 
in toto. While it is only the first stages in differentiation that 
take place, the yolk never being completely absorbed, the most 
