DEVELOPMENT OF ELASMOBRANCH FISHES. 179 



for this stage in general view in PI. XV. fig. 6a, Qh, Qc, in 

 longitudinal section in PI. XV. fig. 7a, 7b, and in transverse 

 section PL XV. fig. 8a — d. The transverse sections are taken 

 from a somewhat older embryo than the longitudinal. In the 

 thalamencephalon there is no fresh point of great importance 

 to be noticed. The pineal gland remains as before, and has 

 become, if anything, 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^ compares the pineal gland with the lono- 

 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 Umbildungsprodukt einer letzten Yerbindung 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 vasculosi are perhaps indicated at 

 this stage in the two lateral divisions of the trilobed ventricle 

 of the infundibulum (fig. 8c). 



The lateral ventricles (fig. 8a) 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. 8a, 

 oil), and contain, as before, prolongations of the lateral ven- 

 tricles. The so-called optic chiasma is very distinct (fig. 8b, 

 op.n)y but the fibres from the optic nerves appear to me simply 

 to cross and not to intermingle. 



The mid-hrain. 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 pro- 

 gressively 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 XV. 

 fig. 4Z>, mh, in which the distinctness of this character is by no 

 means exaggerated). During stage Q it becomes really bilobed 

 through the formation in its roof of a shallow median fun'ow, 

 1 Ent. d. Unke, p. 304. 



