472 C. JUDSON HERRICK 



dorso-median ridge (figs. 73 to 76). The pars ventralis thalami 

 passes over into the pars ventro-lateralis hemisphaerii (primordial 

 corpus striatum) with but little change of structure. The ros- 

 tral boundary of this efferent correlation tissue cannot be fixed 

 definitely; doubtless in that region there is a gradual transition 

 into the adjacent secondary olfactory nucleus. 



The habenula is highly differentiated and separated from the 

 other parts of the diencephalon by a sharp subhabenular sulcus. 

 Extending forward from the habenula, bordering the taenia thai- 

 ami, is the massive dorso-median ridge which crosses the site of 

 the embryonic velum transversum into the telencephalon to end 

 at the neuroporic recess in front of the foramen (fig. 73). This 

 ridge is better developed in Lampetra wilderi (the epistriatum of 

 Johnston, '02) than in most other petromyzonts, but in all cases 

 is an evident structure. In the diencephalon of Ichthyomyzon 

 there is no clearly marked boundary between the dorso-median 

 ridge and the pars dorsalis thalami, these structures being less 

 clearly separate than in Lampetra. On the basis of internal 

 structure I have indicated somewhat arbitrarily a boundary by 

 the dotted lines s.d., on fig. 73. The telencephalic part of this 

 ridge is similarly imperfectly separated from the evaginated hemi- 

 sphere (figs. 75, 76, 77). 



The cerebral hemisphere of the petromyzonts, accordingly, 

 contains no representative of the amphibian pars ventro-medialis 

 and a very small pars ventro-lateralis. By far the larger part cor- 

 responds with the amphibian pars dorso-lateralis and olfactory 

 bulb, which are the direct forward extensions of the pars dorsalis 

 thalami. The rostral end of the dorso-median ridge receives 

 fibers from the olfactory bulb ; it is therefore a part of the general 

 secondary olfactory nucleus. It is traversed for its whole length 

 by fibers of the tractus olfacto-habenularis. Tretjakoff ('09) 

 has shown that these fibers give off collaterals into the dorso- 

 median ridge (his prethalamus) and that some of them end in the 

 thalamus under the habenula. This shows that the epithalamus 

 and dorso-median ridge of cyclostomes are as imperfectly dif- 

 ferentiated functionally as structurally from the pars dorsalis 

 thalami. 



