25O MILDRED HOGE RICHARDS AND ESTHER Y. FURROW. 



is considerable difference in the size of the ganglia of the two 

 sides. Fig. 6 shows a totally eyeless condition on the left side but 

 here the section does not go through the optic tract. In Fig. 7 

 there is no trace of the eye on the left side, but the inner ganglion 

 is shown here and the median in other sections. Figure 8 is a 

 frontal section of a totally eyeless head with no sign of omma- 

 tidia on either side. 



Text-figures 6 to 9 illustrate in diagrammatic fashion the con- 

 dition of the optic tract in normal and eyeless flies. In Fig. 6 the 

 eyes are normal and all three ganglia are present. In Fig. 7 the 

 eyes are reduced but the three ganglia still remain. In Fig. 8 the 

 left side repeats the condition of Fig. 7 but the right side is 

 totally eyeless and only the median and inner ganglia remain. In 

 Fig. 9 both sides are totally eyeless and in both the outer ganglion 

 is absent. 



DISCUSSION. 



It is an interesting fact that the loss of the eyes causes the loss 

 of only the outer ganglion, the two inner remaining, although in a 

 modified condition. From its structure and form we would 

 hardly expect the outer ganglion to remain when the eye is gone. 

 It is so intimately connected to the eye throughout its whole 

 distal surface by fibers which pass through the basement mem- 

 brane that it is difficult to see how the eye could disappear 

 without it. The other two ganglia have obviously a very differ- 

 ent structure. Between either two of these three ganglia the 

 separation might easily occur, but it is evident from the prepara- 

 tions that the break in the optic tract has taken place at the first 

 point where a separation is structurally simple. 



There has been considerable discussion in the past concerning 

 the homology of the optic tract of insects. Berger first pointed 

 out the comparison between the narrow constriction separating 

 the brain and inner ganglion and the entire optic nerve of other 

 forms. The retinular layer he regarded as homologous only to the 

 rod and cone layer of the vertebrate eye. 



Patten however regarded the cone cells as the percipient 

 element and thought them connected to the rhabdome. 

 Lowne conceived of the outer ganglion as the true retina and 

 thought that no fibers pass from the retina through the basement 



