GENERAL ZOOLOGY 



Diencephalon 



Fig. 5.18. ITie early development of the eye. A, B, and C, cross sections through the heads 

 of chick embryos, showing successive stages in the localization of the various parts of the eye. 



the lateral edges of this thicker plate of neural ectoderm, where it is con- 

 tinuous with the thinner superficial ectoderm, folds appear on the surface of 

 the embryo. These are the neural folds, which move toward the dorsal mid- 

 line, where they meet and fuse. This fusion unites not only the edges of the 

 neural plate to form the neural tube but also the edges of the superficial 

 ectoderm that covers the entire surface of the frog embryo and gives rise to 

 the epidermis of the skin (Figs. 5.15/ and 5.17). During the closure of the 

 neural folds some ectodermal cells are left between the superficial ectoderm 

 and the neural tube; they form the neural crest. The neural plate is wider 

 at its anterior end than it is toward the blastopore, and the neural tube is 

 consequently larger at the anterior end. Thus, from a very early period, the 

 anterior region is distinguishable as the part destined to give rise to the 

 brain, and the posterior part is marked as the region which is to give rise 

 to the spinal cord (Fig. 5.19). Within the brain region localized expansions 

 produce first three and then the five brain vesicles which are characteristic of 

 all vertebrate embryos (Fig. 5.21). 



The formation of the eye occurs at the time when the brain vesicles are 

 established in vertebrates. Near the anterior end of the brain, expansions 

 appear to the right and left; these are the optic vesicles (Fig. 5.18). Later 

 the outer cells of these vesicles move in to produce the optic cups. At the 

 same time the superficial ectoderm covering the optic cup thickens and folds 

 in to form the lens vesicle, which is later cut oflT and forms the lens of the 

 eye (see Fig. 4.3, p. 89). 



The first stage in the establishment of the digestive system is seen when the 

 endoderm and archenteron are formed, although the blastopore does not 

 persist as the posterior opening of the alimentary canal. A depression of 

 superficial ectoderm occurs posteriorly to form the proctodeum (Fig. 5.19,4) 

 and anteriorly to form the stomodeum. These pits, lined with ectoderm, be- 

 come the most posterior part of the digestive tract and the mouth cavity, 

 respectively (p. 48). I'he liver and pancreas arise as outgrowths from the 

 archenteron and are lined with endoderm. Closely associated with localiza- 

 tions related to the digestive system are those of the respiratory system. In the 



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