SPINAL CORD REGENERATION. II 423 
A few averaging 5.5 mm. in length were operated in the stage 
having fairly well developed tail buds; a few others at a some- 
what later stage, in which the tail fin was just becoming visible, 
when they averaged 6.5 mm. in length. The operations on the 
5.5 mm. embryos were as successful as on earlier stages, but it 
was found impossible to secure good grafts in the 6.5 mm. stage 
because the voluntary movements of the embryos prevented 
successful healing. This could have been overcome by the use 
of anaesthetics, but as the earlier stages fulfilled the requirements 
of the immediate problem these were not used. 
The operations were performed in 0.4 per cent saline or in clean 
tap-water? under a Zeiss binocular microscope. The dorsum of 
each embryo was cut through in two places, marking out the 
position and length of the piece to be removed. The cuts sev- 
ered the skin, cord and the dorsal portion of the myotomes in- 
volved, but the notochord was left intact, with a few exceptions. 
The embryos were then laid on one side and the two vertical 
incisions joined by cutting horizontally through the skin and 
myotomes of the upper side. The cord was then carefully sepa- 
rated from the notochord, the myotomes and skin of the opposite 
side were cut through and the piece freed from the embryo. 
If the piece of cord were to be reversed, the excised mass was 
turned end-for-end, replaced in the gap in the back of the em- 
bryo and held in situ by piling silver wire about the animal. 
They were allowed to remain thus until the cut edges of the skin 
had reunited (fig. 1, A). This takes from fifteen to twenty-five 
minutes. Of 114 embryos operated, all but 12 were treated in 
this manner. In these 12 the removed piece was regrafted into 
the space from which it had been taken without being reversed. 
In some of the embryos with reversed pieces of spinal cord, the 
wound surfaces were very carefully brought into close apposition 
so that healing per primum resulted. This was presumably the 
condition best adapted to produce a reversal of the polarity of the 
* Clean tap-water has been found to be.as satisfactory an operating medium 
as 0.4 per cent saline (which is isotonic for embryos of this stage) when no very 
great area of epidermis is to be regenerated. Consequently, it was used in all 
but the earliest of these experiments. 
