HISTOGENESIS OF SYMPATHETIC SYSTEM 3 
The embryos of the chick, with few exceptions, were subjected 
to operation at or before the close of the second day (forty-eight 
hours) of incubation. At the close of the second day of incuba- 
tion the spinal ganglia are not yet fully differentiated and in 
the posterior portion of the trunk the neural tube is not yet 
closed. In some instances an attempt was made to destroy 
just enough tissue along the dorsal aspect of the embryo to in- 
sure complete elimination of the neural-crest material and leave 
the ventral half of the neural tube intact. In others, an attempt 
‘was made to destroy sufficient tissue to insure complete elimi- 
nation of the neural crests and the neural tube throughout a 
series of segments or the entire postcephalic region. In a third 
group, the electrode was applied asymmetrically in an attempt 
to destroy the ventral portion of the neural tube unilaterally, 
leaving the neural crests and the dorsal portion of the neural 
tube intact. In a fourth group, an attempt was made to destroy 
the cells which give rise to the sensory ganglia of the vagi and 
the portions of the hindbrain from which the vagi arise. It 
is quite impossible at the time of operation to determine just 
how much tissue is destroyed. In some instances the desired 
result was obtained; in others, either more or less extensive de- 
struction of tissue occurred than was intended. Consequently, 
a series of embryos was obtained in which the destruction of 
nervous tissue varies in extent from the incomplete elimination 
of the neural-crest material to the complete elimination of the 
cerebrospinal nervous system throughout a series consisting of 
few or many segments. 
The rate of mortality among the embryos of the chick which 
were subjected to operation was relatively high and bore a more 
or less definite relation to the extent of the destruction of tissue. 
Nevertheless, a sufficient number of embryos survived to afford 
ample material for study. The majority of those which sur- 
vived were killed at the close of the fifth day of incubation. 
The embryos of the frog were subjected to operation either 
before or immediately after the closure of the neural tube. In 
some the neural crests and the dorsal portion of the neural tube, 
in others, the entire cerebrospinal nervous system was extir- 
