BULLETIN OF THE UNITED STATES FISH COMMISSION. 153 



cord or medullais at first nearly a perfectly solid cord or strand of cells; 

 a canal malies its appearance in its center after the muscnlar somites 

 have been diflerentiated. 



The rudiments of the ears, or auditory organs, in the embryo mack- 

 erel make their appearance soon after the optic cui)s, as slight eleva- 

 tions or welts on each side of the region of the embryonic hind brain, 

 an, Fig. 8. The ridge or welt is simply the lip or proDiinent border of 

 the auditory pit, which is being pushed inward from the outside in a 

 cup-like manner from the inner sensory layer of the epiblast. It soon, 

 however, becomes a closed sack. Fig. 10, au, and by the eighteenth 

 liour two calcareous otoliths are visible in it, as shown in Fig. 12. 

 The complications of structure which develop in the ear beyond this 

 point relate chiefly to the formation of the semicircular canals, and these 

 are developed some time after liatching as ridges or folds on the inner 

 surface of the auditory sack, the walls of which grow inward from above 

 and laterally, joining each other in such a manner that the anterior and 

 l^osterior vertical and horizontal semicircular canals are limited by them; 

 the sacculus and the otoliths lodged in it, consisting of the asterisk and 

 sagitta, finally occupy the lower anterior part of the sack, and the 

 auditory, or seventh nerve, enters it in their vicinity. The auditory 

 sacks, or vesicles, are now almost, or quite, as large as the eyes, and 

 lie on either side of the cerebellum cer, and medulla oblongata mo, as 

 shown in Fig. 17. 



The nasal pits na, Figs. 10 and 12, are at first simple saccular depres- 

 sions diflerentiated from the epiblast in front of the eyes, between the 

 latter and the anterior end of the fore brain. At the age of one week. 

 Fig, 17, na, they are neat, cup-like structures, situated some distance 

 from the edge of the upper border of the mouth just in front of the 

 eyes. At this time it is already possible to demonstrate special so-isory 

 cells in their walls. At a still later period the nasal pits are bridged 

 over transversely by a coalescer' of a part of their opposite edges, so 

 that an anterior and a posterior opening is formed; these communicate 

 with each other beneath the bridge of tissue, and constitute the external 

 nares or olfactory organ of the type characteristic of the true fishes. 

 At what period this last type of structure is developed in the mackerel 

 has not been learned, as it was not formed in the oldest embryos studied 

 by the writer. 



The several portions of the brain begin to be clearly marked ofl" from 

 each other at the eighteenth hour, when the fore-braiu or cerebrum, 

 the mid-brain between the eyes, and the medulla oblongata behind 

 the latter may be distinguished. When the young fish is hatched. Fig. 

 13, all of the divisions may be distinguished, as the cerebellum is now 

 clearly marked ofl" from the medulla. When the medullary groove 

 closed in the region of the brain, a laterally flattened tube was the 

 result, and there is no such extensive anterior downward flexure of the 

 brain on itself as is observed in higher types. As the various constric- 



