CHAP, xx.] SECONDARY SYMMETRY : SCHEME. 



I- I 



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

 surfaces compound together along any very definite line or that the 



l>\\ 



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 -i.lt- 

 of each being indicated by the longer spur. The section of the normal l>r. in 

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

 nearer extra leg in Diagram 1 is shaded, while the rcninti'r is blank. The radii 

 shew them in various positions, anterior, posterior, dorsal, ventral. &c. ivlati\>-ly ti- 

 the normal leg. 



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

 M-, plane of reflexion between the nearer and the remoter extra liiiilis. 



Diagram II is constructed in the same way to illustrate special cases ..f extia 

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

 centrally to the tibial apex each tibial apex is then complete, as on radiu- A <>f 

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

 tibial apices, illustrating how, in cases of complete composition, the extra part- ma\ 

 consist wholly of two morphologically posterior or anterior >urfae.-> arrordnii: a- 

 they arise posteriorly or anteriorly to the normal leg. (See for instance N<>.-. 

 750 and 764.) 







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

 structure of the limbs. The homologous parts seem to be compound-.! 

 at any point, almost as an objrrt partly iimnrrsr.l in mi-miry com 

 pounds with its image along the line to which it is imtii.T-.'d. w 

 ever that line may be. 



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