STRUCTURE OF DOMESTICATED ANIMALS. 305 



various tissues and organs which build up the animal in its 

 perfect condition. 



From the time the chick has reached the condition in which 

 all its organs are fairly sketched, it simply grows larger and 

 larger, and finally breaks through the shell. The skin has 

 already become distinct from the muscles ; the feathers begin 

 to be formed, and all those parts with which you are familiar 

 may readily be distinguished. You see now by what compli- 

 cated process (the details of which I have considerably 

 abridged) this is brought about. 



I have given you but a meagre outline of the changes which 

 take place in the formation of quadrupeds, birds, reptiles and 

 fishes, though this may be sufficient to show that these proc- 

 esses must be studied in every animal independently. 



The figures below, representing a fish in the egg, show at 

 once how difierent the growth of these animals is from that of 



Fig. 17. 

 Young Blenny, copied from Rathke's Embry- 

 ology of the Zoarces Viviparus. Magni- 

 fied. Seen in profile from the right side. 



Fig. 18. 

 The same as fig. 17, seen in profile from the 

 left side. 



Fig 19. 

 The same as figs 17 and 18, seen in front. 



Fig. 20. 

 The same as figs 17, 18 and 19 before the egg- 

 shell has burst. 



the mammalia and birds. Here we have no amnios ; the 

 young fish remains free upon the surface of the yolk. The 

 structure of the body, however, and the circulation of the 

 blood upon the yolk, is strikingly similar to that of the dog, 

 the chicken, or the little turtle. Compare in this respect the 



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