140 



DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



Optic vesicle n^ *5,^ ^i^ ^*>!>^ Optic stalk 



field 



Stage 14 Stage 15 Stage 16 Stage 17 Stage 



Pigmented 



layer '"'^^ ''<^55^^.- Retina Pigmented layer 



Retina 



;\'j- Lens 



- ^^ ^ Optic nerve 



Optic stalk 



Stage 19 Stage 20 Stage 22 Stage 25 



Development of the eye of the frog. 



most immediately, giving rise to paired evaginations known as the 

 optic vesicles, whose walls will give rise to the various ectodermal 

 parts of the eye, exclusive of the lens and cornea. 



Mesencephalon. This is the primary midbrain, or that portion 

 of the primary brain bounded anteriorly and posteriorly by the limits 

 of the dorsal thickening and lines drawn from these limits to the tuber- 

 culum posterius. It is located antero-dorsally to the tip of the noto- 

 chord. 



Rhombencephalon. This is the primary hindbrain, or that portion 

 of the primary brain from the posterior limit of the mesencephalon to 

 the spinal cord which, at this stage, is not clearly separated from the 

 brain. The rhombencephalon lies entirely dorsal to the notochord, 

 and in the frog it is never clearly divided further, as it is in higher 

 forms. 



Derivatives of Forebrain. At this stage of development the fore- 

 brain alone has distinguishing derivatives. Ventral to the notochord 

 there develops a vesicular outpocketing of the floor of the forebrain 

 known as the infundibulum. Its cells will contribute later to the forma- 

 tion of the pituitary gland, in conjunction with a cluster of pigmented 

 ectodermal cells seen between the infundibulum and the roof of the 

 pharynx. This cluster of cells is known as the epithelial hypophysis. 



