654 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



and ventral cell-plate of the chiasma fuse with the post-chiasmatic 

 vertical cell-plate of the lobus ventralis thalami (fig, 7). 



In the later developmental stages the postoptic commissural 

 system appears to increase in extent as compared with the lar- 

 val condition, and the chiasma ridge to push much farther back- 

 ward through the brain substance. In our model its posterior 

 end reaches farther caudad than the transverse plane of the pos- 

 terior end of the primordium hippocampi, while in Johnston's 

 model of the 120 mm. specimen its posterior end lies notably 

 farther forward with reference to both the primordium hippo- 

 campi and the hypothalamic structures. 



The commissures hitherto mentioned lie in the floor-plate of 

 the brain, that is, ventrally of the sulcus limitans of His, while 

 the remaming commissures he in the roof -plate above the sulcus. 



The anterior commissure is a very slender strand of fibers in 

 the lamina terminalis immediately in front of the nucleus olfac- 

 torius medialis (fig. 3, corn.a.) 



The dorsal oljactory commissure (commissura palUi anterior of 

 Johnston '12; commissura olfactoria superior of Sterzi, '07) is a 

 more massive fiber bundle crossing the lamina supra-neuropor- 

 ica immediately in front of the primordium hippocampi (fig. 3, 

 com.d.). 



The superior commissure, or commissura habenularum, connects 

 the posterior parts of the two habenular bodies (fig. 10, com.s.). 



The posterior commissure is very massive, occupying the usual 

 position immediately behind the pineal recess (figs. 3, 11, com. 

 post.). The commissura tecti, which in higher brains occupies 

 the remainder of the roof of the brain is here interrupted by the 

 choroid plexus of the mid-brain roof and its fibers are in part con- 

 centrated in the posterior commissure and in larger numbers at 

 the caudal end of the roof in a massive post-tectal bundle termed 

 the dorsal decussation by Johnston ('02). The post-tectal mes- 

 encephalic decussation (fig. 3, com.p.t.) is continuous behind 

 with the cerebellar commissure, the boundary between these be- 

 ing marked by the decussation of the fourth cerebral nerve. 



