INTRODUCTORY 17 



adaptations to their special circumstances and mode of 

 life. 



The gastrula stage past, all the members of the verte- 

 brate group are still running parallel to one another. 

 In fig. 6 is seen the primitive streak that marks the line 

 where the backbone is to form. In both birds and 

 mammals gill arches appear, showing that their ancestors 



FIG. 6. Embryonic area of a seven days' embryo rabbit. 

 ag', embryonic area ; o, place of future vascular area ; 

 pr, primitive streak; rf, medullary grove. (From 

 Parker and Haswell after Balfour after Kolliker.) 



r ere once " water-breathers," though the arches have ceased 

 to bear functional gills (fig. 7). In both, the heart, that is 

 eventually to be divided into four chambers, begins as a simple 

 tube, but here the record is obscure, and it cannot be said that 

 ft is at any period definitely the two-chambered heart of a fish 

 >r the three-chambered heart of a reptile. 

 This very fragmentary sketch of the parallel development of 



