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 



OL 



OS 



PT 



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. DS, stomodaeal pit. 

 NL, cutaneous or lateral line branch of trigeminal nerve. O C, 

 inner wall of optic cup. O D, outer wall of optic cup. O L, lens. 

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

 jugular vein. 



the gills are developed (Figs, i, K-O, 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 



