THE ECTODERM AND ITS DERIVATIVES 189 



skull and along each side of the spinal cord to receive these con- 

 necting rami from the segmental spinal nerves. Both independent 

 sympathetic nerves and plexi are developed and are found often quite 

 removed from their source of origin. 



The Adrenal Medulla. 



The large brown cells of the inner portion of this endocrine organ 

 arise some time before metamorphosis by migration from the sympa- 

 thetic ganglia and hence are of ectodermal origin. They have lighter 

 staining nuclei and are scattered among the more abundant cortical 

 cells. These latter cells arise from the mesoderm and will be discussed 

 subsequently. 



The Stomodeum and Proctodeum 



The stomodeum and proctodeum are also of ectodermal origin, each 

 being represented by an invagination of ectoderm at either end of the 

 digestive tract. The stomodeal ectoderm meets the oral endodermal 

 evagination to form the oral plate which at the 6 mm. stage ruptures 

 through to form the mouth. Therefore, a portion of the mouth (to the 

 tongue) is lined with ectoderm. The proctodeum is a similar caudal 

 invagination of ectoderm which meets the evagination of the anal 

 endoderm resulting in the formation of a temporary anal plate. This 

 shortly ruptures. The anus is thus partially lined with ectoderm. 



