72 BULLETIN OF THE LABORATORIES 



izontal section at the level of the third and fourth nuclei. A dense 

 nucleus, {N. interpaiheticus,) occupying the region of the raphe caudad 

 to the fourth nucleus is conspicuous. In higher sections the emerg- 

 ence of the fourth root tracts by a number of small bundles describing 

 an abrupt outward curve, is well shown. The interpathetic nucleus 

 above described now lies in the median plane between the nuclei. 

 While no decussation was observed, we are convinced that fibres from 

 the third are given off to tracts passing caudad and laterad. 



The relations of the tegmentum with the corpora quadrigemina 

 may be best studied in longitudinal vertical sections. (Plate XIX, 

 Fig. I.) The nates are nearly (quadrangular and inclined posteriorly 

 while the testes lie nearly at right angles to the former and do not reach 

 one-half the size. Fibres from the optic tracts spread out upon the 

 surface of both anterior and posterior corpora and are followed with 

 difficulty. 



In Didelphys the testes are much more ])rominent than the nates, 

 and contain relatively more gray matter. The tectum proper is not 

 developed below the lateral plane passing through the third nerve nu- 

 clei. In this plane there appear to be five lateral nuclei, three in the 

 thalamus and two in the mesencephalon. The median part of the 

 section is filled with cells, constituting one unseggregated nucleus. 

 We have been able to convince ourselves that the fibres from the optic 

 tracts do not spread out upon the surface of the tectum, but rather are 

 over-capped by a cortex similar to the cerebral. Thus the portion 

 above the aqueduct is separable into four gray layers with three white 

 zones bounding them. The innermost gray zone is immediately sur- 

 rounding the ventricle and is filled with small cells. (Plate IX, Fig. 

 3, etc.) This zOne is limited by the decussating tract of the nates, in 

 which are found the bladder-cells connected with the superior nucleus 

 of the trigeminal. 



The second gray zone seems to partake of the structure of the 

 formatio reticularis and contains scattered nuclei and is bounded per- 

 ipherally by a discontinuous zone of fibres, apparently extending ob- 

 liquely caudad. In connection with these fibres are numerous large 

 multipolar cells, whose processes can be traced in various directions, 

 especially peripherad, for long distances. The space intervening be- 

 tween this tract and the tract formed by the fibres from the optic tract 

 is a reticulum of fibres and cells in concentric and radiating chains. A 

 few of the large multipolar cells are likewise sparsely scattered through 



