Chap. 19 



Mid gut 



Brain 



Future 



mouth 



opening 



Future heart 



DEVELOPMENT 



Nerve cord ^Notochord 



Blastopore 

 osed 



Rectum 



373 



Liver 



Mesoderm 



Brain 



Notochord Nerve cord 



Tail fin 



Future 



mouth 



opening 



Fig. 19.10. Sections of frog embryos, before and after hatching. (From Devel- 

 opment of the Frog, as illustrated by the Mueller-Ward Models. Courtesy, Justus 

 F. Mueller and Ward's Natural Science Establishment.) 



Similar changes take place in the cells of the neural crest as it is transformed 

 into ganglia. 



Sense Organs. An optic vesicle pushes out from each side of the forebrain 

 and makes a well-marked bulge where it is in contact with the skin ectoderm. 

 Each vesicle is shaped like one half of a hollow dumbbell (Fig. 19.11). Its 

 walls are continuous with the wail of the brain, and nerve and sensory cells 

 develop in them. After the vesicle has extended outward, it takes the shape of 

 a double-walled cup. The front or inner wall of the cup will be the sensory 

 layer of the retina containing the light sensitive cells and the cell bodies of the 

 optic nerve fibers; the outer wall will be the pigmented layer. The light-sensi- 

 tive cells develop from cells that were on the former outer surface of the 

 neural folds. Diagrams of cross sections of the same region of the brain and 

 vesicles at successive ages show how the cells originating on the outer surface 

 of the folds are finally located inside the optic vesicle (Fig. 19.11). This ex- 

 plains why light that comes to the retina strikes the nerve cells, and then the 

 sensory cells seemingly wrong end first (Fig. 17.17). As the optic vesicle 

 grows outward, it touches a plate of skin ectoderm which thickens and dips in 

 to make a sac, the lens vesicle, that fits into the cup. The lens vesicle separates 



