Dec, 191G] Profundus and Gasserian Ganglia in Plethodon 27 



Material and Methods. 



The material used consists of a series of embryos of Plethcdon 

 taken from four lots of eggs collected at the same time. There 

 are thirty-eight stages taken at intervals of from 33^ to 1)4. 

 hours, or an average of 4 hours. These range in length from 

 6 mm. to 11.5 mm. All were killed and fixed in Zenker's 

 fluid, cut in transverse sections 10 micra thick and stained 

 with Daelafield's hccmetoxylin and orange G. 



No 30 of this series closely corresponds to the 11.5 mm. 

 embryo studied by Kostir. Of the present series three stages 

 were chosen for detailed study. These are: 



Series : 



M. — 9 mm. in length. 



G. — 7 mm. in length and 263/2 hours younger than M. 



B. — 6 mm. in length and 283^ hours younger than G. 



Three flat reconstructions at a magnification of 150 

 diameters, detailed drawings at magnification of 520 diameters, 

 and outlines at magnification of 80 diameters were made of the 

 selected stages with the aid of a camera lucida. 



The methods of study were in brief as follows : 



After an examination of all 38 stages of the series, three 

 stages as mentioned above, uniform in fixation and staining and 

 exactness of transverse section, were selected as representing 

 the most critical phases in the progressive development of the 

 ganglia in question. The method of reading from later to 

 younger stages was observed and the descriptions of the specific 

 stages are given in this order. The histological characteristics 

 of the ganglia were determined in the later stages since recogni- 

 tion of the ganglia at younger stages depends on a knowledge 

 of the distinguishing features of their nuclei and cytoplasm, as 

 well as of their position. 



In the selection of sections for detailed drawing, care was 

 taken to choose, when possible, comparable levels from the 

 different stages in order to make simpler the comparison of 

 the ganglia at different stages. 



The aim was to choose from each stage one section: (a) 

 near the anterior end of profundus; (b) at the level of the 

 optic stalks; (c) through the roots of the ganglion; (d) through 

 the main portion of the Gasserian, and any other sections 

 necessary to make clear the description. 



