HISTORY OF ZOOLOGY. 



35 



through essentially the stages which remain permanent in the case 

 of lower or at least more primitive animals of the same branch, as 



432 1 



ell 



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

 slits between them : 1, mandibular arch ; 2, hyoid arch ; 3 and 4, first and second 

 gill-arches, a, eye ; n, nasal pit : ft, cardiac region ; e land e II, fore and hind 

 extremities; rn, mesodermal somites. 



FIG. 4. Tadpoles of Rana tempnraria. TM, mouth ; g, upper jaw; 2, lower ja^; , 

 sucking-disc ; hb, external gills ; ifc, region of the internal gills ; w, nose ; a, eye; 

 o, auditory vesicle ; /i, cardiac region ; c/, operculum. 



the three following examples will show: (1) In the early stages of 

 development the human embryo (fig. 3) possesses remarkable 



