496 



MANUAL OF ELEMENTARY ZOOLOGY 



nst. 



cord become connected with the corresponding dorsal roots. 

 The formation of the olfactory organs has been mentioned. 

 The posterior nares arise from the olfactory chambers as down- 

 growths which break through into the mouth. The labyrinth 

 of the ear is formed from the deeper layer of the epiblast as 

 an ingrowth which forms a vesicle, but does not open to the 



exterior. It gradually 

 takes on the shape of 

 the labyrinth by the 

 formation of septa 

 which grow into it 

 and divide it up. The 

 eye has a more com- 

 plicated origin. The 

 retina and the pig- 

 m e n t e d epithelium 

 arise from a pair of 

 outgrowths of the fore- 

 brain, known as optic 

 vesicles, which grow out 

 towards the sides of the 

 head soon after the 

 closure of the neural 

 tube. Each takes on 

 the form of a hollow 



FIG. 376. -A diagram of a transverse bulb on a hollow Stalk, 

 section of the frog embryo at the - 1 ne sta Jk gives rise to 



lr.~ 



-s.d. 



el. 



hatching stage. 



the optic nerve. The 

 outer half of the bulb 

 becomes thickened and 



del., Coslom ; ect., ectoderm; int., intestine ; lr., 

 liver ; m.jrl. muscle plate ; nck. t note-chord ; 

 nst., nephrostome ; s.d., segmental duct; 

 sp.c, spinal cord. The glomeruli are seen then folded back into 

 opposite the nephrostomies. . . . , _ 



the inner half, as a 

 hollow indiarubber ball may be folded when it has been 

 punctured. The two -layered cup which thus arises is 

 known as the optic cup. The thick layer which lines it is 

 the retina, the thin layer on the side towards the stalk 

 is the pigment layer. From the deeper layer of the epiblast 

 there arises a thickening which projects into the mouth 

 of the cup, separates from the epiblast, and becomes the 

 lens, after passing through a stage in which it is a hollow 

 vesicle. 



