FOREBRAIN MORPHOLOGY 403 
cells in which no differentiated nuclei are to be seen. This 
cell-mass is rostrally continuous with the lateral olfactory nucleus. 
The cortex of the tuberculum olfactorium is well developed 
and caudally merges with the preoptic nucleus or the somatic 
area of Johnston. The nucleus olfactorius lateralis is denser than 
in the preceding stage. 
4. Stages of 3.8 to 4.5-cm. body length. In the stages of 3.8, 
3.9, 4, and 4.5 cm. the development is not much advanced in 
relation to the stage of 3.7 em. body length. The brain has grown 
somewhat larger and the cells migrated from the ventricular layer 
are more numerous, but the general arrangement is almost the 
same as before. In the stage of 4.5 cm., however, it is to be noted 
that the pyriform cortex is much better limited against the ven- 
tricular sheet and in the medial part of the forebrain also against 
the general pallium rudiment. In the foremost part of the fore- 
brain, the pyriform cortex and the hippocampal cortex rudiment 
are everywhere continuous with each other (fig. 9). The zona 
limitans lateralis is very conspicuous in the cephalic part of the 
brain (fig. 10, 2.1.1); it grows, however, more indistinct in the cau- 
dal part, behind the olfactory bulb rudiment. A zona limitans 
medialis is not to be seen. 
5. Stage of 4.9-cm. body length. Of this stage I have had at 
my disposal only a series of transverse sections which were in- 
clined forward. ‘Thus the cortical parts of the forebrain are not 
in the same position as in the perfectly transverse sections de- 
scribed before. Therefore, it is somewhat difficult to make an 
adequate comparison between this stage and the preceding ones. 
But so much is at once clear, that no greater changes have taken 
place. The arrangement of the layers is quite the same. The 
part of the primordial cortex, however, uniting the pyriform lobe 
with the hippocampal rudiment has grown more conspicuous, 
a cell-free space in the foremost part of the brain having arisen 
between this cortex and the general pallial rudiment which now 
seems to be in rapid development. 
The evagination of the forebrain being much more advanced, 
the septum has increased considerably in length and the septal 
nuclei are under formation. In the preceding stages the septum 
