THE GENERAL ANIMAL FUNCTIONS. 



79 



Such structures are said to be analogous (as the wing of a 

 bird and the wing of a butterfly). In other cases organs 

 that originate in the same way may have been used so differ- 

 ently as to have a very different appearance, as the various 



FIG. 43. 



FIG. 43. Diagrams illustrating two stages in the development of the vertebrate eye. 

 A, showing the relation of the ectoderm, the brain vesicle, and the optic vesicle. The 

 right side of the figure shows a later stage than the left. B, later stage, showing 

 the lens, eye-ball and retina in position, b.v., brain vesicle formed by the invagina- 

 tion of the ectoderm (ect.) ; I, lens; mes., mesodermal tissue; o.n., optic nerve; o.s., 

 optic stalk; o.r., optic vesicle, a portion of the brain vesicle; r, retinal layer; v.h., 

 interior of eye-ball which comes to contain the vitreous humor. 



Questions on the figures. Which portions of the eye are derived 

 directly from the ectoderm? Which indirectly, i. e., from the brain? 

 Which portions seem of mesodermal origin? By following the invagination 

 by which the retina is formed do you find any suggestion of an explanation 

 of the fact that the sensitive portion of the retina (rods and cones, Fig. 

 173) is directed away from the light? Refer to some work on the embry- 

 ology of the vertebrates for more complete series of figures. 



" legs " of the crayfish, or the wing of a bird and the arm 

 of man. These, notwithstanding their superficial differences, 

 are said to be homologous because of the fundamental equival- 

 ency of structure. 



in. Summary. 



i. Division of labor and differentiation of structure proceed 

 together as the individual develops. All tissues retain the 

 power of using food, of oxidation, of eliminating useless prod- 

 ucts. Other functions incident?.! to these may be relegated to 

 special cells or tissues. 



