SPINAL CORD REGENERATION. I 471 
extent of the skin cut determines quite accurately the amount 
of separation of the myotomes. The division of the notochord 
is the most important criterion of the success of the cut. The 
sections of all embryos which have been operated upon in this 
manner show the exact point of severance of the notochord. This 
is marked by a mass of characteristic cells which have prolifer- 
ated from the cut notochordal sheath. During the earlier stages 
of their growth, the operated embryos present a nick in the dor- 
gal fin which macroscopically determines the location of the cut. 
After operation the embryos were left in the saline for several 
hours. The wound surfaces became covered with epithelium 
in from one to five hours. In all cases the epidermis dipped 
down into the cleft. Until the cut has been covered by the 
migration of the epithelial cells, care must be taken not to shake 
the dish in which the embryos lie, as it is easy to break them in 
two during this period. The wound edges were not apposed to 
one another, with the result that all degrees of gaping of the 
cut were obtained. Individuals in which the cleft in the back 
was more than 0.5 mm. wide were rejected. 
Embryos were killed 1, 2, 4, 7 and 18 days after the operation. 
Sublimate acetic was used as a fixer. Held’s molybdic acid 
hematoxylin and congo red, Ehrlich’s hematoxylin and congo red, 
Mallory’s connective tissue method and erythrosin and toluidin 
blue were used in staining the sections. It was found that Held’s 
hematoxylin gave the best results in the earlier stages and the 
erythrosin and toluidin blue in the older embryos. 
Embryos 24 hours after operation. At this time the embryos 
measured from 3.75 to 4.25 mm. in length and presented a rather 
well developed tail-bud. A deep cleft in the back marked the 
location of the cut. It divided the structures of the dorsum of 
the body nearly to the yolk. Many of the embryos had begun 
to exhibit the voluntary movements which normally appear at 
this time. They consisted in a side to side bending of the neck. 
The portion of the body just in front of the cut was not involved, 
nor did that part of it behind the site of the wound move volun- 
tarily. Light skin stimulation of the body anterior to the cut 
was quickly followed by a movement of the head. Tactile 
