37 



THE PERIPHERAL NERVOUS SYSTEM 



which is present in embryos of 23 mm. The primitive chromaffin cells 

 are small and stain intensely. They continue their immigration until 

 after birth. 



Anomalies. Portions of the suprarenal anlage may be separated from the parent 

 gland and form accessory suprarenals. As a rule, such accessory glands are composed 

 only of cortical substance; they may migrate some distance from their original position, 

 accompanying the genital glands. In fishes the cortex arH medulla persist normally as 

 separate organs. 



sy sy' 



;iv w'?sw: 

 -<?-4*'t 



sy 



FIG. 368. Transverse section through the right suprarenal gland of a 15.5 mm. human embryo 

 (after Bryce). sy, sy', Groups of chromaffin sympathetic cells migrating into the gland. 



E. DEVELOPMENT OF THE SENSE ORGANS 



The sense cells of primitive animals, such as worms, are ectodermal 

 in origin and position. Only those of the vertebrate olfactory organ have 

 retained this primitive relation. During phylogeny the cell-bodies of all 

 other such primary sensory neurones migrated inward to form the dorsal 

 ganglion (Parker) , hence their peripheral processes either end freely in the 

 epithelium or appropriate new cell to serve as sensory receptors (taste; 

 hearing). 



