DEVELOPMENT OF FISH. 445 



their hues, while other secondary sexual characters are devel- 

 oped. The female deposits her eggs either in masses at the 

 surface of the water, as the goose-fish, or at the bottom 

 on gravel or sand as do most other fishes, the male passing 

 over them and depositing his " milt" or spermatic particles. 

 The egg has a thin transparent shell, and the yolk is small, 

 covered with a thick layer of the " white." 



The eggs after fertilization undergo partial segmentation, 

 the primitive streak, notochord, nervous cord, and brain de- 

 velop as in the chick, but that the embryo is to become a 

 fish is soon determined by the absence of an amnion and allan- 

 tois, and by the fact that the germ lies free over the yolk 

 like a band. 



In the pike the heart begins to beat about the seventh day, 

 and by this time the alimentary canal is marked out. The 

 primitive kidneys are developed above the liver. The air- 

 bladder arises as an offshoot opposite the liver from the ali- 

 mentary canal, and the gall-bladder is also originally a 

 diverticulum of the intestine. The urinary bladder in the 

 fish is supposed to be the homologue of the allantois of the 

 higher Vertebrates. The principal external chan ge is the 

 appearance of the usually large pectoral fins. 



The embryo pike hatches in about twelve days after devel- 

 opment begins, and swims about with the large yolk-bag 

 attached, and it is some seven or eight days before the young 

 fish takes food, living meanwhile on the yolk mass. The 

 perch hatches in twelve days after the egg is fertilized, and 

 swims about for eight or ten days before the yolk is absorbed. 

 The gills gradually develop with the absorption of the yolk. 



The tail in most bony fishes is at first protocercal, then 

 becoming heterocercal as in the adult sharks, but subse- 

 quently, after the fish has swam about for a while and in- 

 creased in size, it becomes homocercal or symmetrical. The 

 scales are the last to be developed. 



In the large size of the pectoral fins, the position of the 

 mouth, which is situated far back under the head, the hetero- 

 cercal tail, the cartilaginous skeleton and uncovered gill- 

 slits, the embryo salmon, pike, perch, etc., manifest transi- 

 tory characters which are permanent in sharks. 



