George L. Streeter 339 
has not been observed in brains of other animals. With this in mind the 
writer decided to make the test on some other mammal, fresh embryos of 
which could be easily obtained at all ages, and where control experiments 
could be carried out on embryos of the same litter. The pig was selected, 
and an examination was made of brains of embryo pigs measuring from 
10 to 14 cm. long, which is a period that corresponds to and fully covers 
the time of appearance of cortical papilla in the human brain.’ 
It was found that the pyramidal layer in the pig does not exhibit any 
cortical papilla when carefully preserved; but has always a smooth, regu- 
lar surface, the only indentations being those corresponding to the begin- 
ning fissures, which make their appearance in specimens between 12 and 
13 cm. long. A photograph of a section of a normal brain of a 11.5 cm. 
pig is shown in Fig, 3. 
Having found that cortical papille are not normally present in the 
pig, the next step was to see whether they could not be produced arti- 
ficially, and preferably under conditions which might be probable in case 
of human material. Two possibilities suggested themselves as etiological 
factors; in the first place, maceration of the specimen before it was put 
into the preserving fluid, and secondly imperfect penetration of the pre- 
serving fluid. Under maceration we should have to consider both post- 
partum and intrauterine maceration. The latter might be brought 
about, for instance, by disease of, or abnormal attachment of the placenta 
with consequent disturbance of circulation, and perhaps death of the 
foetus some days before abortion. The second condition, faulty penetra- 
tion of the fixative, might be present in brains of this size were the 
fixative not injected through the arteries or the brain coverings not imme- 
diately opened up so as to permit the direct action of the fluid on the 
brain itself. 
The following experiments were carried out with the idea of imitating 
these two conditions ; on the one hand, for obtaining imperfect penetration 
1The fact that the cortical papille are usually limited to the fourth month 
may perhaps be explained as follows: Up to that time the brain. wall is 
relatively thin and uniform in structure, so that deformities then take the 
form of complete foldings of the wall. In specimens of the fourth month the 
wall is sufficiently thick to prevent foldings of the entire wall, and expansion 
and shrinkage express themselves in a readjustment of its constituent parts, 
some parts being more affected than others. In older specimens such a 
readjustment is prevented by the development of the cell processes and the 
supporting framework of neuroglia, resulting in a structure sufficiently 
firm to preserve its form in the fixative, and consequently no more papille 
or artificial fissures are found. 
