140 DONALDSON. [Vou. IX. 
SERIES III—ConrTinv_Ep. 
AFTER 8 Days. AFTER 24 Days AFTER 41 Days. 
Hemiencephalon, right. . 31.8 36.6 38.7 
i Lefton. 32.5 38.3 38.4 
AN CLAS Cae aaL seen iontte sa 2a = 37-5 —= 38.5 
Encephala with slits . . } oe lee 39:5 
38.5 40.8 40.7 
IAVCTACC™ sheet ast ihe O08 A 1k —— AOL 
In the second series we have a similar relation developed at 
the end of 24 days. 
On considering the third series presented, it will be seen 
that in all cases the slit brains attained a greater weight than 
those intact, and that the hemiencephala occupy in this respect 
an intermediate position. They must of course be considered 
as specimens in which the restraining action of the pia is in 
part removed by the hemisection. 
When the three portions of the third series are compared, it 
is further seen that the intact encephala increase more slowly 
and for a longer time than those which are slit, but that at any 
given date the slit specimen is always the heavier. 
There is no doubt that this pressure of the pia on the 
swelling nervous substance plays an important réle in bringing 
about the differences between the specimens, such as we 
find in our earlier series; but at the same time it is not an 
explanation for all the cases, for in many specimens where 
the weights after hardening were dissimilar enough to excite 
attention, and which were later examined in the light of these 
results, there could not be found a sufficient difference in the 
slitting of the pia to make the above explanation adequate. 
In view of these facts, however, we can understand why a 
small piece of nervous tissue should harden better than a 
larger one, and why the central portion of a large piece, by 
reason of the pressure from the external layers already swollen 
by the hardening fluid on its way to the interior, as well as 
