474 DAVENPORT HOOKER 
movements of the type mentioned above. Tactile stimulation 
of the body behind the cut caused a single reflex response. 
Direct mechanical stimulation of the muscles of the posterior 
half of the body resulted in swimming movements. The loco- 
motion thus produced carried the embryos for considerable 
distances and was sustained for a longer period-than that origi- 
nating from the movements of the anterior end. 
Sections of embryos killed at this stage show that a marked 
advance in development over those killed 24 hours before has 
occurred. The spinal cord and notochord have been here, as in 
‘the earlier stage, completely severed (fig. 2), but the partition 
of epidermis no longer separates the ends from each other. The 
tension. on the skin accompanying the increase in length of the 
tail, noted by Harrison (’98), has pulled the ingrowth of epider- 
mis nearly flush with that of the rest of the body, so that the cleft 
in the dorsal surface of the body is largely limited to the fin. 
The consequent movement in the dorsad direction of the bottom 
of the epidermal partition has drawn with it a mass of con- 
_ nective tissue, derived from the notochordal sheath and attached 
to the under surface of the skin (fig. 3), to a position between the 
severed ends of the spinal cord. This serves as a mechanical 
obstruction to the restoration of the anatomical continuity of the ~ 
cord almost as effective as that previously formed by the epider- 
mal ingrowth. This tissue is in sharp contrast to the mesenchyme 
found elsewhere in the body by reason of its closely interwoven 
fibers and the large number of cells which it contains. It is 
directly continuous with the notochordal sheath on both sides 
of the cut. 
In the spinal cord itself the ventral motor tracts have begun 
to appear. The fibers arise from cells situated in the ventral 
portion of the cord, especially in the anterior half of it. Many 
of the cells lying near the canalis centralis exhibit different 
phases of mitosis. The karyokinetie figures are not localized 
in any particular region of the cord and are no more numerous 
than in normal embryos of the same age. 
Processes extend from each end of the cord toward the other, 
arising from the motor tracts. That they are nerve fibers is 
