304 Illuiois State Laboratory of Natural History. 



somewhat hollowed out wilhiii, is a compact, sohd, whit- 

 ish body. The saccus vasculosus, also of large size, is 

 noticeable from its dark color. It is completely hollow, 

 and its cavit}' communicates with that of the infundi- 

 bulum. In A. rublcundus there is on each side of the in- 

 fuudibulum a conspicuous oval sw^elling, which dissection 

 shows to be a thin-walled sac or pouch opening widely 

 into the cavity of the infundibulum. In Polyodon this 

 swelling is scarcely evident. 



The optic nerves are small in Polyodon, as would be 

 expected from the small size of the eyes. They are of 

 larger relative size in our sturgeon. 



The mid-brain is a compact rounded mass of small 

 size, whitish in color when the pigmented connective 

 tissue is all removed, and without any decided outward 

 trace of separation into two lobes. There is a very 

 slight median impression in front, but be3'ond this the 

 mid-brain shows little appearance of doubleness. either 

 inside or out. In A. ruhicundus this division of the 

 brain is more extensively and deepW impressed along 

 the dorsal middle line, but even in this fish the mid- 

 brain is a single structure with one undivided cavity. 

 Its ventricle is occupied in part by an extension of the 

 cerebellum, called by Goronowitsch the valvula cerebelli. 



The oculo-motor nerve arises on the ventral side of 

 the mid-brain close to its junction with the hind-brain. 



The trochlear nerves are exceedingly small and slender, 

 and their origin is so concealed by the adjacent struc- 

 tures and so covered up by connective tissue that one 

 must follow them up with care in order to find where 

 they leave the roof of the brain. Both nerves, in both 

 Polyodon and A. ruUeundus, seem to arise at the dorsal 

 middle line, and originate so close together that they 

 look like a single nerve passing over the roof of the 

 brain and joined to the latter at the middle line. In the 

 sturgeon there is a small whitish lunate swelling just in 

 front of the point where they leave the brain. 



