BIS TOUT OF ZOOLOGY. 



through essentially the stages which remain permanent in the case 

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



i ;! :.' 1 



Fifi. 3. — Human Embryo, about tilii'd ttr fourrii week. 1-4, visceral arches with f?ill- 

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

 gill-arehes. a, eye ; n, nasal pit : li, cardiac region ; e /and c 11^ fore and hind 

 extremities; »*, meeodermal somites. 



Fig. 4.— Tadpoles of Rana tempararia. m, mouth ; y, upper jaw; z, lower jaw; .'i, 

 suckinE-disc ; Idi. external gills; ih, region of the internal gills ; /(, 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 



