THE FISSURA HIPPOCAMPI 103 
followed from the region of the paraphysis to the end of the 
hippocampal formation. 
The most marked difference. between this embryo and H 
163 (39.1 mm.) is the relative growth of different portions of the 
telencephalon medium. ‘They are as follows: 
1. Increase in length of the lamina terminalis. 
2. No discernible paraphyseal arch. 
3. Increase in the height of the neopallial vault, especially in 
the central region of the hemisphere. 
In the first two brains described, no. 1121 (11.8 mm.) and 
no. 940 (14 mm.), no fissure or sulcus on the medial wall was 
visible. But in the description of nos. H 173 and 460, 19.1.mm. 
and 29 mm., respectively, a shallow groove extended on the medial 
wall of the hemisphere from the region of the future olfactory bulb 
evagination to the most caudal and ventral extremity of the 
lateral area of di-telencephalic fusion. ‘This fissure was also 
noted in much the same position in H 91 and H 41 (27.8 and 31.1 
mm.), although its depth was not as great rostral to the terminal 
plate. Immediately dorsal to the paraphysis this groove is very 
shallow, the wall being almost smooth. But caudal to the para- 
physis the fissure follows the curved contour of the temporal 
lobe of the hemisphere, lying in the medial wall, describing an are 
of a circle. In the oldest brains here considered (39.1 mm. and 
43 mm.), the rostral division of the medial wall is very smooth. 
However, just posterior to the region of the paraphyseal arch lies 
the rostral end of an indentation, the fissura hippocanpi. This 
fissure may be traced as a shallow depression in the hippocampus 
to the caudal end of the primitive temporal pole. 
Besides this fissure another was described, the fissura prima of 
His. It is found only in the older embryos of the series (32.1 
mm., 39.1 mm, and 48 mm.), or in those embryos in which the 
olfactory evagination is prominent. It seems to delimit the 
tissue basal to the attachment of the olfactory bulb from the 
cortex dorsal to its point of evagination. It is the former groove 
or fissure hippocampi which has claimed the writer’s considera- 
tion for a number of years. And you, the reader, are you ques- 
