54 



Vertebrate Embryology 



Even before the tadpole is hatched there 

 may be seen, on each side of the neck region, a 

 series of vertical folds, or thickenings; these 

 are the visceral folds, and it is upon them that 



BF 



OD 



OC 



OS 



TP 



FIG. 20. TRANSVERSE SECTION THROUGH THE 



HEAD OF A TADPOLE 6|^ MM. IN LENGTH ABOUT THE 

 TIME OF HATCHING, THE SECTION PASSING THROUGH 

 THE FORE -BRAIN AND DEVELOPING EYES. (After 



Marshall.) 



A C. carotid artery. B F, fore-brain. Z>5, stomodaeal pit. 

 N L^ cutaneous or lateral line branch of trigeminal nerve. O C, 

 inner wall of optic cup. O Z>, outer wall of optic cup. O Z,, lens. 

 OS, optic stalk. PT, pituitary body. TP, pharynx. VJ % 

 jugular vein. 



the gills are developed (Figs. i,y, Z,, and 22). 

 The external gills appear first, reach their 

 maximum development, and then are replaced 

 by the internal gills. The external gills arise, 

 shortly before hatching, upon the first and 



