192 



THE GERM LAYER DERIVATIVES 



Cerebellum 



Mesencephalon 



Tuberculum 

 posterius 



Epiphysis 



Prosencephalon 

 Optic chiosma 

 Optic recess 



Torus transversus 

 nfundibulum 



Sinus 

 venosus 



Ventricle 



Bulbus arteriosus Hypophysis 



Sagittal section through the anterior end of the 8 mm. frog larva. 



*Choroid coat — thin, vascular layer of mesenchyme immediately around 



the eye. 

 *Sclerotic coat — loose mesenchyme investing choroid layer. 

 *Cornea — extension of sclerotic layer and underlying the epidermis, there- 

 fore both ectoderm and mesoderm. 

 *Eye muscles — small bundles of muscles v^hich control eye movements by 

 way of cranial nerves III, IV, and VI. 

 Ear — arising from auditory placode, then otocyst. 



Utricle — mesial and dorsal, giving rise to the three semi-circular canals, 

 two of which have developed, i.e., anterior dorso-ventral and outer 

 horizontal canals. 

 Saccule — outer and ventral portion of original otocyst. 



Endolymphatic duct — between utricle and brain, joining saccule. 

 Cochlea — mesial portion of saccule, pigmented and ciliated. 

 Nose — arising from olfactory placodes. 



External nares — glandular organ of Jacobson, opened to exterior. 

 Internal nares — extension of tubular opening from external nares into 

 pharynx, called choana. 

 Stomodeum and Proctodeum — ectodermally lined openings into mouth and 

 cloaca. 



"Part or all of the starred structures are derived from mesoderm. 



