chap, XX.] SECONDARY SYMMETRY : SCHEME. 



481 



point of origin is most extraordinary. It does not appear that the 

 surfaces compound together along any very definite line or that the 



II 



Fig. 154. Diagrams of the relations of extra legs in Secondary Symmetry at 

 various positions relatively to the normal leg from which they arise. 



The legs are represented in transverse section, the morphologically anterior side 

 of each heing indicated by the longer spur. The section of the normal leg, in 

 which the radii converge, is shewn with a thick black line. The section of the 

 nearer extra leg in Diagram I is shaded, while the remoter is blank. The radii 

 shew them in various positions, anterior, posterior, dorsal, ventral, iv/c. relatively to 

 the normal leg. 



M 1 , the plane of reflexion between the nearer extra limb and the normal. 

 M' 2 , plane of reflexion between the nearer and the remoter extra limbs. 



Diagram II is constructed in the same way to illustrate special cases of extra 

 legs arising anteriorly or posteriorly. If the two extra legs diverge from each other 

 centrally to the tibial apex each tibial apex is then complete, as on radius A of 

 Diagram I. In Diagram II are shewn two degrees of composition of the two 

 tibial apices, illustrating how, in cases of complete composition, the extra parts may 

 consist wholly of two morphologically posterior or anterior surfaces according as 

 they arise posteriorly or anteriorly to the normal leg. (See for instance Nos. 

 750 and 764.) 



line of division between the several limbs is determined by the normal 

 structure of the limbs. The homologous parts seem to be compounded 

 at any point, almost as an object partly immersed in mercury com- 

 pounds with its image along the line to which it is immersed, where- 

 ever that line may be. 



b. 31 



