4 2 



GENERAL PRINCIPLES OF ZOOLOGY. 



the case of lower or at least more primitively organized 

 animals of the same branch, as the three following ex- 



4321 



el 



ell 



FIG. 3. Human Embryo, about third or fourth week. 1-4, visceral arches with gill-slits be- 

 tween them : i, lower-jaw arch ; 2, hyoid arch ; 3 and 4, first and second gill-arches. 

 a, eye ; , nasal pit ; /i, cardiac region ; e I and e //, fore and hind extremities ; m, 

 mesoblastic somites. 



FIG. 4. Tadpoles of Rana temporaria. i, mouth; -, upper jaw ; z, lower jaw ; s, sucking 

 disk ; 6, external gills ; ik, region of the internal gills ; , nose ; a, eye ; o, auditory 

 vesicle ; h, cardiac region ; d, operculum. 



amples will show: (i) In the early stages of development 

 the human embryo (Fig. 3) possesses remaikable resem- 



