416 EMBRYOLOGY. 



CHAPTER XVI. 

 THE ORGANS OF THE OUTER GERM-LAYER. 



THE outer germ-layer has for a long time also borne the name 

 dermo-sensory layer. By this its two most important functions are 

 both indicated. For in the first place it forms the epidermis together 

 with its various products, such as hair, nails, scales, horns, and 

 feathers ; and in addition various kinds of glands : the sebaceous, 

 sweat- and milk-glands. Secondly, it is the matrix out of which 

 the nervous system and the most important functional parts of the 

 sensory organs, the optic, auditory, and olfactory cells, are derived. 



I begin with the most important function of the outer germ-layer, 

 the development of the nervous system, then proceed to the develop- 

 ment of the organs of sense (eye, ear, and organ of smell), 'and finally 

 discuss the development of the epidermis and its products. 



I. The Development of the Nervous System. 

 A. The Development of the Central Nervous System. 



The central nervous system of Vertebrates is one of the organs 

 first established after the separation of the germ into the four 

 primary germ-layers. As has already been stated, it is developed 

 (fig. 41 A) out of a broad band of the outer germ-layer (mp), which 

 stretches from the anterior to the posterior end of the embryonic 

 fundament and lies in the median plane directly above the chorda 

 dorsalis (ch). In this region the cells of the outer germ-layer grow 

 out into long cylindrical or spindle-shaped structures, whereas the 

 elements occurring in the surrounding parts (ep) flatten out and 

 under certain conditions become altogether scale-like. Consequently 

 the outer germ-layer is now divided into two regions into the 

 attenuated primitive epidermis (Hornblatt) (ep) and the thicker 

 median neural or medullary plate (mp). 



Both regions are soon sharply separated from each other, since the 

 neural plate bends in a little (fig. 41 B) and its edges rise above the 

 surface of the germ. In this way there arise the two medullary or 

 dorsal folds (mf), which enclose between them the originally broad 

 and shallow medullary or dorsal furrow. They are simply folds of 

 the outer germ-layer, formed at the place where the neural plate is 

 continuous with the primitive epidermis. They are therefore com- 

 posed of an outer and an inner layer, of which the inner belongs to 



