358 J. B. JOHNSTON 
A tract from the medial olfactory nucleus over the foramen inter- 
ventriculare to the nucleus habenulae is not known in other verte- 
brates. The fibers passing up from the medial olfactory nucleus 
as far as the primordium hippocampi occupy the same place as 
the tractus olfacto-corticalis septi in selachians and amphibians. 
If these fibers end in the primordium hippocampi, then what 
appears to be the continuation of them to the nucleus habenulae 
must be classed as cortico-habenular fibers. A second bundle 
comes from the lateral and caudal walls of the hemisphere, passes 
up behind the foramen through the inner and lower part of the 
primordium hippocampi to join with the first as it enters the 
nucleus habenulae (figs. 17, 18, 19). This is the tractus olfacto- 
habenularis of authors. A third bundle comes from the preoptic 
region and joins the first two (fig. 35). These three bundles unite 
into a large compact bundle which traverses the eminentia thalami 
at the bottom of the prehabenular recess and enters the nucleus 
habenulae (figs. 19, 26, 27). The fourth component of the stria 
medullaris is very diffuse and comes from the whole thickness of 
the primordium hippocampi. Axones arising from the cells of 
the primordium are seen in many cases passing at first peripherally 
among the dendrites of these cells and then turning to run toward 
the nucleus habenulae through the substance of the primordium 
hippocampi. It is these fibers in addition to the first and second 
bundles above described that give a longitudinal striation to the 
whole of this body. At the caudal end of the primordium these 
fibers enter the nucleus habenulae caudal and somewhat dorsal 
to the compact bundle of the stria medullaris (figs. 16, 17). 
The left nucleus habenulae is very small in Lampetra and nearly 
all the left stria medullaris enters the superior commissure. In 
the preparations from which figures 14 to 22 were drawn no fibers 
were seen ending in the left nucleus. The view expressed in the 
writer’s paper on Acipenser (’01, p. 115) that the larger size of 
the right nucleus is correlated with the ending of a larger por- 
tion of the tractus olfacto-habenularis in the right nucleus is 
strongly supported in Lampetra. The superior commissure pre- 
sents two main divisions, a more rostral, compact bundle and a 
more caudal portion made up of several strands (figs. 17, 18). 
