510 JULIA B. PLATT. 
able positions confirms the evidence given by the depth of the 
median ridge (fig. 9), by its far-reaching prolongation into the 
underlying tissues (fig. 12), and its frayed edge, making it 
probable that the median and transverse ridges of the trunk 
throughout their length are the source of cell proliferation 
from the ectoderm, and of addition to the mesectoderm. 
When the neural Anlage has extended over several segments 
of the trunk, and lies above the spinal cord as a band of tissue 
five or six cells wide if measured from one lateral margin to 
the other, the even surface of the spinal cord is interrupted at 
the point where the motor nerve is to be formed by the out- 
growth of a fine protoplasmic prolongation, which soon after 
its appearance becomes attached to one or more of the neigh- 
bouring mesenchyme cells. These cells, moreover, send 
prolongations to meet the spinal outgrowth, as seen 
in fig. 10, which passes through the root of the third spinal 
nerve. 
Somewhat later, but before the cells of the neural crest in 
their downward course touch the root of the motor nerve, a 
condition represented in figs. 16 and 17 is found. There is as 
yet no indication of the formation of a “‘ Randschleier,” and in 
both sections cells are seen to migrate from the spinal cord. 
Fig. 17 passes through the root of the third spinal nerve in the 
younger embryo, and fig. 16 through that of the tenth in an 
older embryo. Fig. 13 shows a further stage in the develop- 
ment of the motor nerve in which its main path is already 
established by the bipolar prolongations of a medullary cell. 
The section passes through the root of the fifth nerve in the 
same series from which figs. 14, 15, and 16 are taken. 
Figs. 14 and 15 cut respectively the right and left roots of 
the fourth spinal nerve, and show that the cells which mi- 
grated from the spinal cord have now taken on the fibrillar 
striation that belongs to nerves. The nucleus of the cell 
which lies in the nerve path is entirely enclosed by the striated 
protoplasm of the nerve, in the threads of which the yolk 
granules seem entangled. Although, in this later stage, the 
cells of the neural mesectoderm have come in contact with the 
