2. Work on the Microsoopical Anatomii of the Brain. 



Anatoniical work on the nervous system of the Selachii was 

 begun relatively early, but aiicrosoopical study lagf^ed somewhat 

 behind that on the bony fishes. One of the earliest researches 

 touching the onicroscopical structure of the selachian brain was 

 that of Leydig ('5?). This versatile investigator was engaged 

 in tracing the general organogeny and histology of the rays 

 and sharks, and so his work upon the brain was not special in 

 its character. Here, however, he discovered the olfactory 

 glomeruli, a result altogether sufficient in itself. Of course, 

 the nature of the microscopical methods then in use did not 

 permit Leyaig to see much more than the general outline of a 

 glomerulus and the fibres associated therewith. It remained 

 for later workers to trace the full significance of his discov- 

 ery. 



The two decades following the research of Leydig witnessed 

 the unfolding of the germ of a special neurological technique. 

 Stilling had introduced the method of studying the brain by 

 means of sections as early as 194?, but the great advantages 

 to be derived from staining the sections were not realised 

 until 1R59, when Gerlach soaked his sections in a solution of 

 carmine. Later, 137?, Gerlach obtained such brilliant results 

 with gold chloride as to lead to many trials with this reagent, 

 while some of the possibilities of osmic acid wei-e also becom- 

 ing known. Finally, chromic acid and the bichromates had come 



