HISTORY OF ZOOLOGY 



27 



and are functionally intelligible in fishes hut they are not compatable 

 with a lung-breathing mammal and must undergo fundamental changes 

 to become of use. They are, like so many other structures in the human 

 being, not to be understood from present functions, but must have an 

 historical meaning. (2) Frogs in their tadpole stage have an organiza- 

 tion similar to that which is permanent in certain Amphibia, the Per- 



3 



FIG. 3. FIG. 4. 



FIG. 3. Human embryo, 4.2 mm. long (after His). Pericardium and lateral 

 body wall opened, yolk-sac and allantois cut away, course of blood-vessels shown; a, 

 arterial trunk; a/, allantois stalk; c, precava, uniting with yolk and umbilical veins; d, 

 intestine; do, yolk stalk; h, ear vesicle; A", ventricle of heart; o, upper jaw; r, olfac tory 

 groove; s, tail; n, lower jaw; us, somites; V, atrium of heart; 1-5, the five arterial arches. 



FIG. 4. Tadpoles of Rana temporaria. in, mouth; g, upper jaw; z, lower jaw; 

 s, sucking-disc; kb, external gills; ik, region of the internal gills; n, nose; a, eyes; 

 o, auditory vesicle; h, cardiac region; d, operculum. 



ennibranchiata (fig. 5), which stand lower in the system; they have a 

 swimming tail and tuft-like gills, which are lacking in the adult frog. 

 (3) There are certain parasitic Crustacea, which live upon the gills of 

 fishes, and seem not at all like their relatives. They are shapeless masses 

 which were formerly regarded as parasitic worms. Their systematic 

 position was only determined by their embryology (fig. 6). Here it is 



