IOWA ACADEMY OP SCIENCES. 
151 
Second . — Formaldehyde has an application in those methods 
used for tracing the course of medullated nerve fibers. All 
such methods, whether the original Weigert or some modifica- 
tion of it, are usually long and tedious, the time required fre- 
quently being some months. This length of time is often a 
very serious objection. Formaldehyde can be introduced in 
these methods for the purpose of rapidly giving firmness to the 
nervous tissue, and then subsequent steps may follow in quick 
succession. In this way the time may be reduced to ten days 
for the whole process. 
Third . — In the study of nerve cells formaldehyde may now 
claim a place in the beautiful impregnation method of Golgi. 
The application is made in Golgi’s “rapid” method, and con- 
sists in the substitution of pure formaldehyde for the 1 per cent 
csmic acid of the hardening mixture. The advantages result- 
ing from this substitution may be an increased clearness of the 
subsequent silver impregnation, or in the slightly wider lati- 
tude of time during which hardening may occur. The physio- 
logical condition of the nervous tissue appears to be a very 
important factor in all Golgi work; and perhaps formaldehyde 
is less sensitive to these differences than osmic acid. However 
that may be, osmic acid in this method cannot be dispensed 
with. Workers should use both hardening mixtures side by 
side. The results attained by one will supplement those of the 
other in a most valuable way, thus virtually doubling the effi- 
ciency of the study as a whole. 
THE NERVE CELLS OF THE SHARK’S BRAIN. ^ 
BY GILBERT L. HOUSER. 
The sharks are of the greatest interest to the morphologist 
on account of the many ancestral characters of their organiza- 
tion. The researches of recent years indicate that they 
represent quite well the primitive stem of the jaw-bearing 
vertebrates. With this fact in mind, the importance of the 
study of the shark’s brain is at once apparent. For obvious 
* The following brief notes are to be considered as in the nature of a mere pre- 
liminary communication on this subject. 
