No. 2.] DEVELOPMENT OF PETROMYZON'. 257 



ispheres, but in the adult they are considerably larger, a growth 

 which is probably to be referred to the extraordinary size and 

 complexity of the olfactory apparatus in the adult. Both the 

 olfactory lobes and the hemispheres are at first solid masses of 

 nerve-cells, closely packed together and without any appear- 

 ance of fibres. After the formation of the ventricles, a lining 

 layer of epithelial cells forms around them, and considerable 

 bundles of fibres make their appearance. In larvae of 40 to 

 50 mm. in length, the minute structure of these bodies is very 

 much as in the adult, consisting of nerve-cells imbedded in 

 fibrous bundles. Towards the centre the nerve-cells are most 

 closely crowded, gradually separating towards the circumfer- 

 ence, which is formed by a thin layer of white matter. 



The relative position of the lamina termmalis changes with 

 advancing age. At first it is in close contact with the nasal 

 epithelium, and thus remains for a considerable period (Figs. 2 

 and 3, PL VIII; Fig. 11, PI. IX), the hemispheres extending 

 but little in advance of it. The formation of the olfactory lobes 

 is accompanied by a depression of the lamina (Fig. 16), and as 

 the lobes increase in size, the lamina is carried further and fur- 

 ther away from the nasal capsule, and the depression becomes 

 more marked. I cannot state at what period the anterior com- 

 missure between the hemispheres first makes its appearance, 

 but it is very distinctly marked in larvae of medium size (60 

 mm. and upwards). 



The development of the tJialamencepJialon presents several 

 points of considerable interest. At the time of hatching this 

 region has walls of considerable thickness, with much thinner 

 roof and floor ; in the median line, both dorsal and ventral, 

 there is but a single layer of cells ; the ventricle is quite widely 

 open, and especially expanded above the middle of its height. 

 In later stages the lower part of the ventricle becomes much 

 narrower and is reduced to a m.ere slit, while the upper portion 

 increases in width, and, as a whole, the thalamencephalon be- 

 comes very much higher and narrower. The roof of the ven- 

 tricle thins out, and is pressed down by the overlying epiphysis 

 and its accompanying structures, while the thalami increase in 

 height and thickness and gradually overtop the cerebral hemis- 

 pheres. Fibres appear in this region much later than in the 

 mid and hind-brains, so that in young larvae the thalamen- 



