338 Proceedings of the Royal Society of Edinburgh. [Sess. 
seen in some embryos, but it was not until some hours later that a satis- 
factory demonstration of these nerves was obtained. 
In the tissue lying round the notochord, and between it and the aorta, a 
very few nerve cells were recognised in the thoracic and lower cervical 
regions. These cells closely resembled those described at the end of the 
second day as having broken away from the posterior ganglia. From their 
appearance and position they seemed to represent a stage in a ventral 
migration of nerve cells from the posterior spinal ganglia to form the 
sympathetic chain. At this stage no satisfactory chain of cells could be 
traced from the posterior spinal ganglia to the neighbourhood of the noto- 
chord or aorta, but as the number of cells found in this latter position was 
so small, it was highly probable that the relationship of the connecting cells 
was lost in the various sections. Careful examination of the tissue in front 
of the aorta and of the splanchnopleure failed to show the presence of 
either nerve cells or fibres. 
At the end of the third day both anterior and posterior spinal nerves 
were readily recognised. 
In the tissue round the notochord, and between it and the aorta, nerve 
cells were seen similar in appearance to those described in this position at 
the middle of the third day (PI. I. fig. 2). These nerve cells, which are 
somewhat more numerous than at the middle of the third day, were found 
practically only in the thoracic and lower cervical regions. At several 
points in the thoracic region an almost complete chain of cells was traced 
from the ventral extremity of a posterior spinal ganglion to the neighbour- 
hood of the notochord. None of these nerve cells was found in front of the 
aorta, and no nerve elements were found in any part of the alimentary canal. 
After four and a half days incubation the sympathetic system as a 
whole was found to have made a very marked advance in development. 
The sympathetic chain was represented by a fairly uniform chain of cells 
lying behind the aorta (PI. III. fig. 1). In the upper abdominal region nerve 
cells were traced round the aorta to its ventral wall, in front of which a 
delicate plexiform arrangement of sympathetic nerve cells and their out- 
growths was formed. This plexus was stained a fainter colour than the 
nerve cells in the sympathetic chain. 
In the mesentery a thin chain of nerve cells and fibres led down to the 
gut wall, while in both the stomach and upper intestinal walls a very 
delicate plexiform arrangement of nerve cells and fibres was recognised 
(PL III. fig. 2). In the stomach wall the nerve cells were arranged in 
two layers, one on the outer margin of the wall, while the other was much 
more deeply situated. 
