48 M. L. Shorey 
From these two experiments, and others which are not so con- 
clusive, but which furnish evidence corroborating these results, I 
think it is safe to conclude that even when the primordium of all 
the musculature of a single somite is destroyed, some motor cells 
and motor fibers will develop within the medullary tube of this 
segment. ‘They will be fewer in number than on the normal side, 
they will extend a shorter distance from the spinal cord, and instead 
of forming a nerve trunk with well-defined boundaries, in the initial 
stages, they will run freely into the mesenchyme, forming an 
irregular mass. ‘Their behavior in later stages of development is 
shown by the experiment which follows. 
Experiment 706. ‘Vhe period of incubation was 50 hours, and 
an attempt was again made to destroy the 17th, 18th and 19th 
somites. [he embryo was examined and preserved g2 hours 
later. Figs. 11 and 12 give the external appearance. ‘There is a 
slight elevation in the anterior part of the wing region on the 
operated side, and examination of sections shows that a portion of 
the 17th myotome is left, the 18th is entirely wanting, and only a 
fragment of the 1gth is present. No brachial plexus was formed 
on the operated side, and each nerve must therefore be treated 
separately. The 12th ganglion is smaller than on the normal side, 
as is also the corresponding ventral horn, and the course of this 
nerve, the first brachial, is shown in Fig. 42,4. There is no distinct 
ganglion for the somite following this, but in the place where it 
should appear there is a clear space much the shape of a ganglion 
(Fig. 21 ), containing occasional cells which appear to be of a 
ganglionic character. These become more numerous posteriorly 
and finally blend with the 14th ganglion, which is abnormal in 
shape. ‘The interpretation of this would seem to be that the cells 
of the 13th ganglion, which would probably have been partially 
formed at the time of the operation, have been attracted to the 
region of the 14th, and have united with it. There is no distinct 
nerve root in this somite, but there is a motor nucleus and most of 
the fibers arising from these cells pass out at various levels of the 
spinal cord to join the third brachial nerve. A few fibers end freely 
in the mesenchyme, as in Experiment 69, a short distance from the 
spinal cord. The ventral horn of the third somite is smaller on 
