338 The Ohio Journal of Science [Vol. XVI, No. 8, 



posteriorly while the lengths of the ganglia diminish. Thus, 

 there is, progressively, a greater extent of contact between the 

 placodes and ganglia in proportion to the length of the ganglia. 

 This is evidence of the greater maturity of the more anterior 

 ganglia, which is in accord with the general law of antero- 

 posterior differentiation. 



TABLE I. 

 (Showing length of ganglia and contact area in 20 mm. embryo.) 



Gang. Length of Ganglia Length of Contact Area Ratio 



VII 340 microns 10 microns 1-34. 



IX 190 " 40 " 1-4.75 



Xi 270 " 90 " 1-3. 



X2 130 " 90 " 1-1.46 



X3 150 " 120 " 1-1.25 



X4 210 " 160 " 1-1.4 



Table II shows the length of the area of contact in the 18 

 mm. stage. The development has not progressed to the degree 

 seen in the older stage and, consequently, there is less difference 

 in the lengths of the contact areas in the younger stage than 

 in the 20 mm. stage. The placodal contributions are propor- 

 tionately much larger when compared to the size of the general 

 visceral portions than in the older series. This is evidence that 

 the total absence of the darkly-staining cells in the 22 mm. 

 embryo, examined in connection with other work in progress in 

 the department, may be interpreted as meaning, only, that 

 these cells, in the latter case, have undergone complete meta- 

 morphosis and mersion with the general visceral cells so that 

 they do not stain differently. This is also the probable expla- 

 nation of the failure to distinguish them in Rana (Landacre 

 and McLellan '12), since this form undergoes relatively more 

 rapid development and the process was completed in the earliest 

 stages studied. 



TABLE II. 



(Showing length of contact area in 18 mm. embryo.) 



Gang. Length of Contact Area 



VII ISO microns 



IX 160 



Xi 100 



X2 90 



X, SO " 



X4 100 



