460 SIMON H. GAGE 
3. Glycogen is, then, a builder as well as an energy producer 
for nervous as for all other forms of tissue. 
4. Its appearance in developing tissue in all forms of verte- 
brates depends in part, at least, upon the relative time in which 
the tissues must function. For example in Amblystoma that 
must have full functional activity very early in its life, the 
perfecting glycogen appears correspondingly early, while with 
the chick it is late in appearing. 
5. After reaching their definitive form the elements of the 
nervous system in the lowest vertebrates, Amphioxus and 
larval lampreys (Ammocoetes), continue their glycogenic func- 
tion in the central nervous system. In the adult lampreys (Pet- 
romyzon and Lampetra), this function persists throughout life 
in the nerve cells of the retina. With the higher vertebrates, 
glycogen in demonstrable amount is not found in the nervous 
system after the embryonic period, the liver and muscles then 
assuming the main glycogenic function. 
That is, specialization of this function keeps pace with dif- 
ferentiation of structure consequent upon advance in ‘the 
zoological scale. 
METHOD OF DEMONSTRATING GLYCOGEN 
The fundamental thing is that no liquid is to be used in any of the 
steps that will dissolve the glycogen. The most certain medium for 
fixing is alcohol. Absolute alcohol is mostly recommended; but, as 
glycogen is wholly insoluble in alcohol of 67 per cent and, of course, 
in all stronger grades, one has considerable range. 
As alcohol diffuses through the tissues slowly, only small animals 
and small embryos should be fixed entire. For the organs and tissues 
of larger forms small pieces or widely opened and dissected organs in 
which the aleohol comes quickly in contact with all the parts containing 
glycogen should be used. Plenty of aleohol should be used—fifty 
times the bulk of the tissue—and it is well to change it two or three 
times. In general it is safer to use alcohol of 95 per cent, then it is 
not liable to be diluted by the lymph sufficiently to make the glycogen 
soluble. 
As alcohol distorts the tissues, it is well to carry along parallel 
specimens prepared by the usual fixers. Mercuric chlorid, or mercuric 
chlorid and dichromate mixtures are good. Picric alcohol is also good 
and it has the advantage of fixing the glycogen as well as the other 
tissue elements (it is compbdsed of 67 per cent alcohol, 500 ec.; picric acid, 1 
