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 



NL. 



Fig. 20. — Transverse section through the 

 head of a tadpole b'/i mm, in length about the 

 time of hatching, the section passing through 



THE FORE - BRAIN AND DEVELOl'ING EYES. (After 



Marshall.) 



A C, carotid artery. B F, fore-brain. D S^ stomodaeal pit. 

 N Ly cutaneous or lateral line branch of trif^eminal nerve. O C^ 

 innerwall of optic cup. C^Z*, outer wall of optic cup. (9/,, lens. 

 OS, optic stalk. FT, pituitary body. TP, pharynx. Vy, 

 jugular vein. 



the gills are developed (Figs, i, K-0, 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 



