THE GENERAL ANIMAL FUNCTIONS 85 



113. Analogy and Homology. — In comparing various ani- 

 rhals we find that they may do the same work with organs that 

 arise in very different ways, which, however, because they are 

 adapted to perform similar tasks, look somewhat alike. 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 differently as to have a 

 very different appearance, as the various "legs" of the crayfish, 

 or the, wing of a bird and the arm of man. These, notwithstand- 

 ing their superficial differences, are said to be homologous because 

 of the fundamental equivalency of origin and structure. 



Pig. 45. 



Pig. 45. 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 invagination of the ectoderm (ec/,); I, lens; mes,, meso- 

 dermal tissue; o.n., optic nerve; o.s., optic stalk; o.v,, optic vesicle, a portion of the brain vesicle; 

 ft 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. 175) is directed away from the light? Refer to 

 some work on the embryology of the vertebrates for more complete series of figures. 



114. Differentiation and Reintegration of Parts. — The chap- 

 ter thus far should have given the student a fair appreciation 

 of the degree to which the work of a complex organism is divided 

 up among the organs and of the differences found in the organs 

 themselves because of this. We all know that this differentia- 

 tion makes for efficiency. There are two sides to this, however. 



