Differentiation of Neuroblasts 43 
ocular micrometer. The effect on the ganglia is more evident, and 
the loss is distributed as follows: 
DORSO-VENTRAL LATERAL 
DIAMETER — DIAMETER 
PER CENT PER CENT 
Crimalon Posucandoootodod0oor no pou nGonBnaTebOSndtae Ieee. 5 Oc Onne I 9 
(Ganyahom iting oaviacbudanopdecob0 ds soo oHOREE Uae OnoC Oo Iun OOo ne ae MRO: II 
Gray pli carts renate eget ctate lal eieietetcter-Laieve stele s/e1/etn/sie/e)«)=ieraleinlni-ta'~/efols\eloialeiolel= ate Ricks 
The areas within the spinal cord are not sufficiently differ- 
entiated to make méasurements practical, but Fig. 22, a photo- 
eraph of the spinal cord in the region of ganglion 13, shows a loss 
in the ventral horn. The posterior horn seems to be entirely 
unaffected. 
Fig. 23 is a cross-section of the spinal cord in Experiment 79, 
which was preserved, at the same age as Experiment 404, and 
agreed with it in all essentials. Other specimens at this stage of 
development showed the same conditions. 
In Experiment 142, killed 24 hours after the operation, in which 
when examined there was no external evidence of the wing except 
a slight thickening of the body wall, I am able to demonstrate 
but a very slight difference in the development of the nervous sys- 
tem on the two sides, and this is found in a decrease in the extent 
of the peripheral branching and a very slight decrease in the size 
of the ventral horn. At this period the beginning of the differen- 
tiation of bones and muscles is evidenced by aggregations of cells 
into more dense masses, but no individual structures can be iden- 
tified. The nerve trunks have reached the inner margin of the 
wing, and are beginning to branch within it. “These conditions are 
illustrated by Figs. 13, 39 and 24. 
Other embryos killed at this period always show a decrease in 
the extent of the branching of the nerves, but in some cases there 
is no appreciable effect on the spinal cord. 
E. Experiments at Later Stages of Development 
The next experiments were designed to test the effect of remov- 
ing the wing at different stages of its development. I am indebted 
to Professor Lillie for materials for Experiments 5 and 10, the 
