506 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [52] 



difficult to see this part of the brain distinctly in living embryos, on 

 account of their transparency and the consequent impossibility of mak- 

 ing out its limits with distinctness, surrounded as it is by structures 

 very similar in optical character. 



In a few instances I have succeeded in getting horizontal sections 

 through the plane of the optic crus of embryo fishes. It presents noth- 

 ing different, at a relatively late stage, from what is seen in the structure 

 of the same part in the adult. The development of the cerebral hemi- 

 spheres seems to take place relatively late in embryonic life, and they 

 also retain a remarkable solidity. The cerebral hemispheres grow some- 

 what in size after the larval period of development is past, but in most 

 Teleostei they never attain the dimensions of the mid-brain. 



12. — The olfactory and auditory organs. 



At a comparatively early stage of development the nasal organs are 

 differentiated from a pair of small circular tracts of the sensory layer 

 of the epiblast lying between the anterior extremity of the neurula and 

 the optic vesicles as shown at na in Figs. 26 and 27. At a late stage, 

 with the advance of development and the consequent growth and shift- 

 ing of the parts in relation to each other at the forepart of the head, the 

 nasal involutions or sacks are displaced downwards so that as the head 

 of the embryo grows in thickness and juts forward more, they are car- 

 ried down nearer to the point where the mouth will open. They thus 

 finally come to lie nearer the margin of the upper jaw than would be 

 supposed possible, judging from their original position. ' They are at 

 first simple thickenings of the sensory layer ; by the fourteenth day iu 

 the young cod they have been completely involuted as thick saccular 

 depressions continuous at their borders with the skin. The thick co- 

 lumnar epithelium which clothes them is the olfactory or Schneiderian 

 membrane, which during the later larval or even, in most cases, proba- 

 bly, the post-larval stages become involuted into folds which have a 

 radial arrangement on the floor of the nasal sack, with secondary ridges 

 connecting them. During this time also the nasal membraneous bridge 

 is developed across the pit, in consequence of which it acquires ventral 

 and dorsal openings which communicate with each other. The bridge 

 itself may in some cases be developed as a considerable flap-like exter- 

 nal process, or there may be a considerable space intervening between 

 the dorsal and ventral openings of the nasal organ. The other changes 

 which the olfactory organs undergo during the post-larval development 

 of the young fish are principally those of position and changes iu the 

 length of the olfactory nerve. This last is at first extremely short. I 

 believe it to originate primarily from the upper hinder portion of the 

 neurula destined to form the cerebrum, from what Marshall and Balfour 

 have called the neural crest ; at any rate, there is much to favor this 

 view from what we may learn from an examination of stages such as 

 those represented in Figs. 26 and 27. As development proceeds, its 



