216 THE DEVELOPMENT OF THE TADPOLE. 



In the words of Balfour — " The gulf between the two types of 

 limbs is so great that there is room for a very great diversity of 

 opinion as to the mode of evolution of the cheiropterygium." 

 Gegenbaur, Huxley, Gotte, and Balfour all differ seriously from 

 each other in their explanation of this question of evolution, the 

 only point on which they appear to agree being in the statement 

 that undoubtedly, in some way or other, not easy to be explained, 

 the fore-limb of vertebrates was evolved from the fin of a fish. 

 Why not the reverse ? appears to me to be a question quite as 

 dififtcult to answer, 



EXPLANATION OF PLATES XXL, XXII. 



The whole of the figures on these plates illustrate the development of 

 the limbs of the Frog, from their earliest stage to the period at which 

 they become functionally active. 



Plate XXI. 

 Fig. I. — Shows the posterior portion of the body of a Tadpole on 

 March 23rd, about a month after laying, with a few cells in 

 the ectoderm from which the leg is developed. 

 ,, 1(1. — Some of the same cells on a larger scale. 

 ,, 2 and 2a. — The same, a week later, shewing the connection 



between the muscles of the tail and those of the abdomen. 

 ,, 3. — The whole animal, shewing the position of the fore and hind 



legs at a more advanced stage. 

 ,, 3a. — The hind leg, h.L, of Fig. 3 more highly magnified, shewing 

 the commencement of the formation of the pigment cells. 



Plate XXII. 

 ,, 4, 5, 6. — Hind legs in further stages of development, shewing 

 points of ossification and the commencement of the forma- 

 tion of muscle. 

 ,, 7. — Hind legs fully developed, shewing bones with their attach- 

 ment, position of the urostyle, with the acetabulum, and the 

 princi]>al muscles. Date, July 4th. 

 ,, 8. — Fore leg at same date as hind leg, 3a. 

 ,, 0. — Ditto, slightly more developed. 



,, 10. — Ditto, nearly developed, but not yet extruded beyond the 

 skin. 



yk., yelk. p.c, pigment cells. 



i., intestines. ect., ectoderm. 



h.L, hind leg. sc, scapula. 



t., tail. fi., fore-leg. 



hit., humerus. ph., phlanges. 



7mi. , muscles. fe. , femur. 



H. , notochord. ma., manus. 



ep., epidermis. os., points of ossification. 



ti., tibia. ac, acetabulum. 



p., pubis. ta., tarsus. 



