444 APPLIED BIOLOGY 



the egg-cells (one sperm for each egg), and fertilized them. 

 Later, the fertilized eggs are placed in boxes arranged so 

 that water runs over them while they develop. After 

 hatching, the young fish (called fry) are kept in small pools 

 or tanks, where they are easily fed and protected until large 

 enough to care for themselves in rivers and ponds where their 

 enemies live. Of course many are killed after they are 

 turned loose, but a far larger number of eggs may develop 

 into adult fishes if hatched under artificial conditions than if 

 allowed to be laid where fishes will naturally deposit them. 



364. Embryology of Higher Vertebrates. The early 

 stages of the embryonic development of reptiles, birds, and 

 mammals have a general similarity to those of the frog ( 59) . 

 In all cases, the fertilized egg-cell divides into numerous 

 cells which then form the body of the embryo. 



It is a significant fact that there is great similarity in the 

 early stages of all vertebrates. This is illustrated by Fig. 

 150, in which in parallel columns are early, intermediate, and 

 late embryos of a fish, a salamander, a reptile, a bird, and a 

 mammal. In the early stages there is so great similarity that 

 only specialists in zoology could distinguish between these 

 embryos ; but as development proceeds there is more and 

 more differentiation, and the final stages at birth or hatching 

 are easily identified as fish, bird, etc. 



One of the most remarkable facts connected with this 

 similarity of embryos of different classes of vertebrates is the 

 presence in all of them of certain structures which are use- 

 less in the higher forms. Most striking of such useless struc- 

 tures are the gill-slits. In fishes these openings from the 

 pharynx to the exterior are exits for water which enters at 

 the mouth. Between the slits are the gills, through whose 

 delicate membranes blood circulates and exchanges oxygen 

 and carbon dioxide with the water. Thus in fishes the gill- 

 slits are useful as part of the respiratory system. 



The gill-slits develop in very young embryos of fishes and 



