456 Animal Biology 



four pairs of incompletely formed slits which somewhat resemble the gill- 

 bearing arches of a fish (Figs. 224 to 227 and 363). There are numerous 

 blood vessels here, but no true gills. These gill arches give rise to such 

 structures as the following: from the arches arise muscles used in chew- 

 ing foodj middle ear bones, hyoid bone (at base of tongue), certain 

 facial nerves and muscles, part of the cartilage of the larynx and its mus- 

 cles; from the slits between the arches arise such structures as the Eusta- 

 chian tubes, external ear passage, part of the tonsils, thymus, and para- 

 thyroids. 



conjunctiva — 

 aqueous humor, 

 crystalline lens 

 lower lid — — 



vitreous humor — 

 retina ' — - — " 



pigment layer — 



optic nerve — • 



Fig. 227. — Stages in the development of the eye in embryos of a vertebrate ani- 

 mal shown somewhat diagrammatically. A, Thickening of the ectoderm of the 

 side of the head as the optic cup begins to form on the end of the optic stalk; 

 B, formation of the lens and the retina in the optic cup; C, later stage with parts 

 labeled. (From Atwood and Heiss: Educational Biology, The Blakiston Co.) 



QUESTIONS AND TOPICS 



1. Describe the frog egg before development begins. Where and when can you 

 find such eggs? Why are such eggs not deposited in the fall? What is the 

 relationship between the dark pigment of the eggs and the source of heat 

 energy for the development of the eggs? 



