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 

 r>ensory 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. TJte Development qf the Central Nervous System. 



The central nervous system of Vertebrates is one of the organ- 

 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) t whirh 

 stretches from the anterior to the posterior end of the embryonic 

 fundament and lies in the median plane directly above the clun la 

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

 out into long cylindrical or spindle-shaped structures, whi'iv;i> tin* 

 elements occurring in the surrounding parts (ep) flatten out ai.l 

 under certain conditions become altogether scale-like. Consequently 

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

 Attenuated primitive epidermis (Hornblatt) (ep) and the thirk.-r 

 median neural or medullary plate (nip). 



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 moduli aiy <>r 

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

 and shallow medullary or dorsal furrow. They are simply fold 

 the outer germ-layer, formed at the place where tli neural plat i\ 

 continuous with the primitive epidermis. They are therefore com- 

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



