( 38) 



plane in the rostral part of the tectum (region of the commissura 

 post). Fig. 19 gives a representation of this. It is very noticeable — 

 more than in echidna — that the nucl. magnocellularis tecti consists 



^^^m 





Fig. 18. Echidna hyslrix. Fig. 19. Didelphys marsupialis. 



of 2 varieties of elements, viz. rounder, oval, vesicular and lumpy- 

 polj'edrical. If tig. 19 (didelphys marsupialis) be compared with (ig. 12 

 (chelone midasj the resemblance between the opossum and the turtle 

 is directl}' seen. A third marsupial of which the Brain Institute 

 possesses a sagittal series affords a view of the similarity in proximo- 

 distal measurement between its tectal V nucleus and that of chelone 

 or varanus (fig. 20, onychogale frenata, compared to fig. 15, Nile 

 varanus). The course and the exit of the mesenc. V fibres exhibit 

 no special feature. In the region of the motor V nucleus the mid- 

 brain root loses to it several fibres, the others make a distal convex 

 bend and join to the most ventral part of the rad. spinalis trigemini 

 already entered into the oblongata, distinctly separate from, and 

 dorsally to, the motor V root {Macropus rohustiis fig. 21). 



All the other mammals more or less show in principle the following 

 features in the distribution of the cells on a cross plane as well 

 as in sagittal direction viz. 1, the cells fall awa}^ from the tectum, 

 along the boundary of the central grey substance ventrally round 

 the aqueduct ; 2. the cells proceed, as it were, distally in the direction 

 of their neurones and come to lie more or less close to the cross 

 plane of the motor V nucleus. This latter case is well seen in fig. 22, 

 a sagittal section laterally to the ventric. quartus, through the 



