THE PELAGIC STAGES OF YOUNG FISHES. 5 



figs. 2, 3), and at the time of hatching extends as a median vertical fold 

 along the whole dorsal line, returning on the ventral side as far as to the 

 yolk-sac. (PL II. fig. 3, PL 111. fig. 8, PL X. fig. 4, PI. XL fig. 8. PL XII. figs. 

 6 3 7, PL XIII. figs. II. L2, PL XIV. figs. G. 7, PL XVI. figs. 7, 8, PL XVIII. 

 fig. 6, PL XIX. figs. 1. 5.) 



With the resorption of the yolk-mass is connected the disappearance of 

 the embryonic fin fold in advance of the vent, and usually at that time (PL II. 

 tig. 12, PL III. figs, lo, 11. PL XIII. figs. 14, 15, PL XV. figs. 13, 14) the 

 pectorals are well developed and have become a powerful limb in the young 

 embiyo. The resorption of the yolk-mass is also accompanied by a length- 

 ening of the intestinal tract, and the formation of a larger alimentary canal 

 and of a swimming-bladder. (PL II. ficr. 12, PL XI. figs. II, 12. PI. XIII. 

 figs. 13-15, PL XV.. PI. XVI. figs 8-10. ) In some cases (PL II. fig. 3, PL XII. 

 figs. G, 7, PL XIII. figs. 11-14, PL XVII. fig. 2, PL XVIII. fig."o), where the 

 vent when the young fish is hatched is not marginal, but lateral, this gradu- 

 ally moves down toward the edge of the embryonic anal fin. In the very 

 3'oungest stages, immediately on leaving the egg, the embryo depends mainly 

 upon its embryonic dorsal and ventral (its leptocardial fin) lor locomotion. 

 The propelling powers of this fin are proportionally very large, and at no 

 time of its life is the young fish better provided with means of locomotion, or 

 with organs oi' sense to detect the most minute changes in the medium sur- 

 rounding it. Compare, for instance, the muscular axis of a newly hatched 

 embryo (PI. II. fig. 3, PL X. lig. 1. PI. XII. figs. G-S, PL XIII. fig. 12, PL XVI. 

 fig. 6, etc.), the size of the embryonic fin, the comparatively great bulk of the 

 brain and of the chorda, the immense size of the eyes, the great develop- 

 ment of the lateral organs, and the size of the auditory capsules, with those 

 of a more advanced embryo (PL XI. fig. 12, PL XV. fig. 11), and then with the 

 older adult stages of these same fishes, and we cannot fail to be struck by 

 the contrast they present. Everything in the young embryo seems emi- 

 nently adapted to enable it to receive the most delicate impressions, whether 

 ii be from its exquisitely sensitive lateral line or from its huge eyes; and its 

 gigantic embryonic fins enable it with comparative ease to move rapidly away 

 from disturbing influences, to say nothing of their extreme transparency and 

 their power of reducing the pigments which cover them to a minimum while 

 in rapid motion, and thus readily to escape from their enemies. 



With the growth of the embryo the young fish depends more and more 

 for locomotion upon the use of its pectorals, ami these develop quite rapidly, 



