CH. XV] ORGANS FROM THE ECTODERM 163 



leaves the capsule. These auditory vesicles separate from the 

 surface ectoderm. "At the time of the separation the vesicle 

 is a closed sac somewhat pyriform in shape; its lower or 

 ventral portion being spheri- 

 cal and lying opposite the 

 notochord, and its dorsal 

 wall being prolonged up- 

 wards into a short blind 

 diverticulum lying at the 

 side of the hind-brain. The 

 wall of the vesicle consists 

 of a single layer of cubical 



or columnar cells." This ^^ 



ectodermal sac becomes the FIG. 50. Cross-section through hind- 

 sensory lining of the inner SchSd. "^ ^^ *" (E) ' N ' 

 ear (Fig. 50). 



THE NERVES 



At the time when the medullary plate forms as a thickening 

 of the ectoderm, there also forms, as we have seen, on each side 

 of the plate a lateral neural ridge or plate of ectoderm. Each 

 neural ridge appears at first as a continuation of one side of 

 the thickened medullary plate (Fig. 26). A slight constric- 

 tion on each side marks the line of demarcation between the 

 medullary plate and the neural ridge (Fig. 42). The neural 

 ridges are more conspicuous at the anterior end of the medul- 

 lary plate ; they also develop somewhat earlier in this region. 

 After the medullary plate has rolled up to form the medullary 

 tube, the lateral neural ridges are also carried up, retaining for 

 a time their primitive connection with the outer (now dorsal) 

 part of the medullary tube (Fig. 40). 



The neural ridges next become broken up into a series of 

 dorsal nerves, the cells collecting at certain regions, and thin- 

 ning out and disappearing in the intermediate regions. The 

 dorsal nerves grow down later between the myotornes and the 

 nerve-cord. Accumulations of cells occur at certain regions 

 on each dorsal nerve to form the ganglion of the dorsal root, 

 and nerve-fibres are spun out from the cells of the ganglion. 

 The ventral roots of the spinal nerves appear much later. 



