286 



VARIATION AND MONSTROSITIES. 



Fig. 191. — Diagram of a degenerating limulus embryo, showing the order in which the paired organs unite in 

 the. middle line and then disappear. Organs formed by normal apical growth are represented by small letters; 

 the unpaired organs formed by their union, and which are about to disappear, are in capitals. 



Fig. 192. — Diagram illustrating the law of growth and degeneration of segmented, bilaterally symmetrical 



animals. 



A. The segmental organs, seen in mercator projection from the neural surface, are represented by letters, and 

 the metameres by numbers. A-P, The principal, or neural axis; A, cephalic; P, caudal end. A, m.n., area of 

 initial growth; m.L.R.n, area of multiple growth; L.R.P., area of diminishing growth. 



B. The arrangement of organs after the degeneration of the area in figure A, enclosed by the lines A.m. a,n.* 

 With the disappearance of this area, the lines m.a." and a.M.'' unite to form the line of unpaired organs E-A. 



