CEREBRAL COMMISSURES IX THE VERTEBRATA. 



467 



unnecessary to describe them in detail. But there arc certain features in the region of 

 the recessus superior which reproduce tlie condition found in the Sauropsida more nearly 

 than does that of the adult Monotreme : in addition, the hippocampal formation is very 



Kk. 14. 





recs. 



C.a/r-m l— 



CM 



Greatly enlarged representation of jiart of a coronal section of the brain of OrjiitJiorhijnchus, to show the recessus 

 superior and the adjoining structtires. The letters utarr/. refer to the fasciculus marginalis, labelled /««. in the 

 other figures. The adjoining diagram is an enlarged part of fig. 11, to explain the jilane (x-if) in which the 

 section was cut. 



mucli simpler than it is in the adult, and its exact relations to the j'araterminal body 

 less ambiguous. Such a section is roughly represented in figure 15, and the regions 



Fig. 1.5. 



rec s 



fd. 



c.d vM. c.v -^ p<a.r<a.. 



A scheme of a coronal section of the brain of a very young h'cJiiiliKi, greatly enlarged. 



surrounding the recessus superior have been reproduced again upon a larger scale in 

 another figure (16). The former illustration needs no further description, and I may 

 turn at once to the consideration of figure 16. 



In many minor points the condition in the foetal Echidna ditfers from that found in 

 the adult Ornithorhy minis (and Ecliidnu). Instead of the lateral waUs of the superior 

 recess being slightly thickened witli grey matte)- derived from the paraterminal body, 



