204 



MORPHOLOGICAL DEVELOPMENT. 



but little from triple bilateral symmetry, the deviation be- 

 comes great as we ascend. Figs. 273 and 274 show how, 

 besides being divisible into similar halves by a vertical plane 

 passing through its axis, a Fish is divisible into halves that 

 are not very dissimilar by a horizontal plane passing through 

 its axis, and also into other not very dissimilar halves by a 

 plane cutting it transversely. If, as shown in Figs. 275 and 

 276, analogous sections be made of a superior Eeptile, the 

 divided parts differ more decidedly. When a Mammal and a 



Bird are treated in the same way, as shown in Figs. 277, 

 278, and Figs. 279, 280, the parts marked off by the dividing 

 planes are unlike in far greater degrees. On considering 





