TYPES OF VERTEBRATE AND INVERTEBRATE EYES II 



(Fig. 5, p. n). A pit, the optic groove, then appears on each side in 

 the situation of the future optic vesicle (Figs. 6, 7, 8). Later, when the 

 medullary folds close in to form the neural tube, the optic pits appear 

 as hollow evaginations springing from the sides of the neural tube. The 



N PI 



.CE. 



Fig. 5. — Transverse section of Open Neural Plate of Rana palustris, near 

 its Anterior End, showing the Position of the Optic Rudiments. E., 



ALREADY MARKED OUT BY THE FORMATION OF PIGMENT. 



CE. : Cutaneous epithelium. N.Pl. : Neural plate. 



(After Eycleshymer, 1895, Textbook of Embryology, Vol. II, Graham Kerr.) 



expanded outer end of each outgrowth is known as the primary optic 

 vesicle, and the constricted neck is the optic stalk. The stalk is at first 

 a hollow tube which connects the cavity of the primary optic vesicle with 



OpGr 



Fig. 6. — Model of the Head of a Pig Embryo 47 mm. (16 days), showing the 

 Optic Grooves on the Surface of the Open, Neural Plate. The Model 

 is viewed from the Front and Left Side. (After Froriep.) 



Op.Gr. : Optic grooves. 



the third ventricle ; and its opening into the ventricle corresponds to 

 the mouth of the original pit or groove which opened on the surface of 

 the open medullary plate. The epithelium, which forms the outer part 

 of the primary optic vesicle and later becomes invaginated to form the 



