164 BULLETIN : MUSEUxM OF COMrARATIVE ZOOLOGY. 



my results as set forth in my preliminary papers (Sargent, :00, :01). 

 The numerous other writers on the selachian brain have failed even to 

 mention this great nidulus of cells. 



B. Observations. 



1. Raja. Of this family, I have studied the optic reflex apparatus in 

 several species, Raja erinacea, R. ocellata, and R. laevis, and at various 

 stages of development, from the newly hatched larva to tlie adult. There 

 are no essential differences between these species, but the following 

 description applies more particularly to R. erinacea, unless otherwise 

 stated. 



a. Morjj/iolof/y of the MesertcejjJialon. The morphology of the mesen- 

 cephalon is relatively simple. The optic lobes are vi'ell developed, nearly 

 hemispherical, and separated by a shallow median dorsal fissure (Plate 2, 

 Fig. 12) which marks the primitive median zone. The thick walls of the 

 optic tectum have encroached upon the median plane, so that the median 

 zone has lost its primitive essentially ependymal structure, and, though 

 thinner than the lateral portions of the tectum, is crowded with gan- 

 glionic cells (Plate 2, Fig. 12 ; Plate 3, Fig. 15), the most conspicuous of 

 which are the great cells of the 'Dachkern.' The dorsal decussation 

 of the mesencephalon carries a strong bundle of nerve fibres from the 

 stratum medullare i)rofundum of either side transversely across the 

 median plane immediately above the ' Dachkern' (Figs. 12, \5,dec. d.). 



In the newly hatched skate, 10 cm. long, the region about the pos- 

 terior commissure remains in a relatively primitive state, reminiscent 

 of the condition in cyclostomes. The posterior commissure is clearly 

 marked off from the tectum, and in the median plane is connected with 

 it by only a narrow bridge of tissue (Plate 3, Figs. 15 a, IG). A long 

 narrow recessus extends above the commissure and between it and the 

 tectum opticum. Into this recessus is continued the thick ependymal 

 surface layer of the pars intercalatus, lining it completely. This cpen- 

 dyma is continued from its dorsal and posterior surface as a cone-like 

 prominence {eras, e^ end., Fig. 15 ^; Plate 11, Fig. 71). 



In the adult skate the increase in the thickness of the tectum and 

 the posterior commissure has resulted in a more intimate consolidation 

 of these two structures (Plate 3, Fig. IG). The recessus between them 

 has become considerably changed and elongated. It has a long narrow 

 neck expanding into a recess triangular in sagittal section of the brain 

 (Fig. 16). Its anterior dorsal portion is prolonged into two conical 

 lateral horns, similar to the corresponding structures in Petromj^zon, and 



