GENERAL LAWS OF ANIiMAL STRUCTURE. 



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accepted. The more probable view is that the same fold 

 of the skin which formed the lower unpaired fin (anal 

 fin) divided as it came forward into two lateral folds 

 (Fig. 168, A), that these by local enlargements became 

 the paired fins, anterior and posterior (Fig. 168, B), and 

 by addition of skeletal framework were developed into 



Fig. 16S. — Diagrams showing probable origin of limbs : A, earliest stage, 

 showing lateral folds ; B, later stage, after paired fins were formed (from 

 Wiedersheim) ; C, the formation of limbs from fins (from Parker). 



limbs (Fig. 168, C) and finally became connected with 

 the skeletal axis for more effective action. The subject 

 is, however, still obscure. 



Serial Homology of Other Systems. — Other sys- 

 tems show segmented structure, but not so clearly. 

 Next to the skeletal in this regard comes the nervous 

 system. Thus we may conceive the cerebro-spinal axis 

 as a series of ganglia (fused, however, into a continuous 

 tract) corresponding to the vertebras, and giving off each 

 a pair of nerves; and, according to the vertebral theory 



