70 A HALF-HOUR WITH THE 



of a bone will frequently serve to indicate whether 

 an animal belonged to fishes, reptiles, birds, or 

 mammals. This is a matter of importance to the 

 geologist in determining the character of the in- 

 habitants of the earth at former periods of its 

 history. 



The shell of eggs seems to be formed on the same 

 general principles as other hard parts, and the 

 tendency to the formation of canaliculated cells 

 may be easily observed (Fig. 177, PI. 6). The 

 young egg-shell should be examined (&), if the 

 object is to study the history of the development 

 of the shell ; and this may be compared on the 

 one hand with the shells of the Mollusca and the 

 Crustacea, and on the other hand with those of 

 the scales, teeth, and bones of the vertebrate 

 animals. Egg-shells present very different ap- 

 pearances. The shell of the emu, for instance, 

 exhibits a series of dark triangular spots (Fig. 178, 

 PL 6). 



As one of the hard parts of animals, the struc- 

 ture of cartilage is very interesting. A slice may 

 be obtained from the gristle of any young animal. 

 Its structure is best seen in the mouse's ear (Fig. 

 221, PI. 8). No one who looks at this object can 

 but be struck with its resemblance to vegetable 

 tissue ; and it was this resemblance which led to 

 the application of the cell theory of development, 

 which had been made out in vegetable structures, 

 to those of animals. 



Many of the soft parts of animal tissues afford 

 instructive objects under the Microscope. If the 

 tongue is scraped, and a drop of the saliva thus 

 procured placed under the Microscope, it will be 

 found to contain many flat, irregular, scale-like 

 bodies with a nucleus in the centre (Fig. 4, PI. 1). 

 These scales are flattened cells, and closely re- 



