78 



EVOLUTION AND GENETICS 



centers also appear. The bones formed from these four centers 

 are the anterior pubic bones and the posterior ischia, between 



Fig. 53. — Series illustrating a theory of the development of the pelvic girdle, 

 a, sturgeon, Acipenser; b, a ganoid fish, Scaphyrhynchus; c, a ganoid, Polyp- 

 ierus; d, a primitive salamander, Necturus; e, a South African frog, Daclyl- 

 elhra; f, turtle. In a the part m is formed by a fusion of the anterior rays. 

 The pieces kk, segmented off from m in b, form in c a rhomboidal plate. 

 In d this i)late has grown large and bears a pair of ossified ilia, i, and a pair 

 of centers of ossification, the is(;hia, h. In e two more centers of ossifica- 

 tion, the pubes, g, have appeared. ' f, is a t /pical pelvic girdle with all its 

 parts. The epipubis, e, is incidental and unimportant. (From Wilder's 

 History of the Human Body, with the permission of Mrs. H. H. Wilder and 

 Henry Holt and Company.) 



which in reptiles develop the obturator foramina which sometimes 

 join to form a single large opening. While pronounced modifica- 

 tions of these bones occur in higher forms, such as the ventral 

 separation of the ischia in man and of both ischia and pubes in 



