CHAPTER XV 



ORGANS FROM THE ECTODERM 



THE outer covering-layer of the embryo, the ectoderm, gives 

 rise to the nervous system and organs of special sense (eyes, 

 ears, nose). The adhesive glands or "suckers" are also formed 

 by this layer ; and the anterior and posterior divisions of the 

 digestive tract, the stomodreum and proctodasum, have a lining 

 of ectoderm. 



In the present chapter we shall follow the development of 

 these organs. 



THE CENTRAL NERVOUS SYSTEM 



The medullary plate appears on the surface of the young 

 embryo at the time when the yolk-plug is about to be drawn 

 in from the surface. It extends over about one-third of the 

 circumference of the egg and is, at first, quite broad. It is 

 slowly converted into a tube by the drawing together of its 

 material, and by a subsequent over-rolling of its sides to meet 

 in the mid-dorsal line. This change into a furrow, and then into 

 a closed tube, involves extensive movements of the material of 

 the plate. Whether the plate moves as a whole, or whether 

 the movement is only the sum-total of changes in shape and 

 position of the individual cells, is not known (compare Figs. 26, 

 42, 50). While the medullary tube is developing, the embryo 

 as a whole is changing its spherical shape into a more elon- 

 gated form and the medullary tube is also drawn out. 



The medullary plate is formed, for the most part, from a 

 thickening of the inner layer of the ectoderm (Figs. 26 and 

 42). It is continuous on each side with a broad flange or ridge 

 of thickened ectoderm (Fig. 42, Nc). This ridge of cells, the 

 neural crest or ridge, is also lifted up during the formation of 



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