402 DEVELOPMENT OF ELASMOBRANCH FISHES. 



development of the brain. Its structure is represented for this 

 stage in general view in PI. 16, figs. 6a, 6b, 6c, in longitudinal 

 section in PL 16, figs, ja, jb, and in transverse section PL 16, 

 figs. Sa d. The transverse sections are taken from a some- 

 what older embryo than the longitudinal. In the thalamerTce- 

 phalon there is no fresh point of great importance to be noticed. 

 The pineal gland remains as before, and has become, if any- 

 thing, longer than it was, and extends further forwards over the 

 summit of the cerebrum. It is situated, as might be expected, 

 in the connective tissue within the cranial cavity (fig. Sa, pn], 

 and does not extend outside the skull, as it appears to do, 

 according to Gotte's investigations, in Amphibians. Gotte 1 

 compares the pineal gland with the long persisting pore which 

 leads into the cavity of the brain in the embryo of Amphioxus, 

 and we might add the Ascidians, and calls it "ein Umbildungs- 

 produkt einer letzten Verbindung des Hirns mit der Oberhaut." 

 This suggestion appears to me a very good one, though no facts 

 have come under my notice which confirm it. The sacci vas- 

 culosi are perhaps indicated at this stage in the two lateral 

 divisions of the trilobed ventricle of the infundibulum (fig. Sc). 



The lateral ventricles (fig. Sa) are now quite separated by a 

 median partition, and a slight external constriction marks the 

 lobes of the two hemispheres ; these, however, are still united 

 by nervous structures for the greater part of their extent. The 

 olfactory lobes are formed of a distinct bulb and stalk (fig. Sa, 

 ol.l), and contain, as before, prolongations of the lateral ventricles. 

 The so called optic chiasma is very distinct (fig. 8^, op.ti], but 

 the fibres from the optic nerves appear to me simply to cross 

 and not to intermingle. 



The mid-brain. The mid-brain is at first fairly marked off 

 from both the fore and hind brains, but less conspicuously from 

 the latter than from the former. Its roof becomes progressively 

 thinner and its sides thicker up to stage P, its cavity remaining 

 quite simple. The thinness of the roof gives it, in isolated 

 brains of stage P, a bilobed appearance (vide PL 16, fig. 4^, mb, 

 in which the distinctness of this character is by no means 

 exaggerated): During stage Q it becomes really bilobed through 



1 Ent. d, Unke, p. 304. 



