SYMPATHETIC SYSTEM IN MAMMALS 219 



Various methods of technique were employed, but the iron- 

 ha?matoxylin method was found to be most satisfactory. The 

 embryos were fixed in Zenker's fluid, chrom-aceto-formaldehyde, 

 or chrom-oxalic acid. The sections were usually cut a thick- 

 ness of 10 microns. It was found most satisfactory to over-stain 

 in hematoxylin, then to differentiate in the iron-alum bath until 

 the color had almost disappeared except from the denser tissues, 

 and to counter-stain lightly with orange-G. The degree to which 

 the haematoxylin shall be differentiated in order to obtain the best 

 results must be learned by experience. 



The results obtained from one lot of embryos may be of inter- 

 est to students of special technique. A few young embryos taken 

 from the laboratory collection were sectioned and stained. These 

 had been kept in 10 per cent formaldehyde for some ten years. 

 When stained by the iron-hsematoxylin method, the nuclei of the 

 nerve cells took a much deeper and more solid stain than in fresh 

 material. These preparations were, therefore, found very service- 

 able in tracing migrant nervous elements. 



The silver reduction method was used with good results in 

 the later stages. 



IV. OBSERVATIONS 

 I. SYMPATHETIC TRUNKS 



(a) Early development. — The earliest traces of the sympathetic 

 trunks appear in the thoracic region of embryos of the pig about 

 6 mm. in length, as small cell-aggregates lying along the sides of 

 the dorsal surface of the aorta. The spinal nerves in the thoracic 

 region have already become fibrous and extend peripherally to a 

 point a little beyond the dorsal level of the aorta. Fibrous com- 

 municating rami are not present as yet, and the sympathetic 

 anlagen are apparently independent of the spinal nerves. 



In embryos 7 mm. in length, the anlagen of the sympathetic 

 trunks may be traced throughout the thoracic and the dorsal 

 region. The cell-aggregates have become larger, and because 

 of the strong curvature of the embryo they are brought into such 



