20 Charles Russell Bardeen 



spinal cord may be followed well into the tail. It gradually 

 becomes narrow and more rounded as one passes posteriorly. The 

 neural canal is relatively large. 



Organs of Special Sense — The nasal pit is connected with the 

 pharynx by a solid column of cells. The thickened lateral wall of 

 the brain projects against the olfactory pit and the olfactory nerve 

 is in process of formation. The lens of each eye is in con- 

 tact with the ectoderm. Although differentiation of the lens is 

 well under way it still contains a vesicular cavity. The sensory 

 layer of the retina is thick and closely applied to the pigment 

 layer. As mentioned above, the optic stalk near the optic cup is 

 small and seems to contain nerve fibers. 



Each auditory vesicle is a simple closed sac with a short blind 

 diverticulum. The auditory ganglion consists of a mass of cells 

 situated between the anterior portion of the auditory sac and the 

 medulla. 



Peripheral Nerves — The trigeminal ganglion is large. It is not 

 in connection with the ectoderm. The ganglion of the facial 

 nerve cannot be sharply distinguished from that of the auditory 

 nerve. The glossopharyngeal and the vagus ganglia are well de- 

 veloped but it is difficult to trace the chief branches of these nerves 

 for any considerable distance. The spinal ganglia consist of well 

 marked groups of cells lying ventro-lateral to the spinal cord. 

 The nerve fibers of the spinal nerves are beginning to form in the 

 proximal spinal segments. 



Alimentary Canal — The shallow stomodaeum opens into the 

 pharynx. The lips and jaws of the stomodaeum are beginning to 

 grow forward. 



The broad pharynx communicates by gill slits with the exterior 

 The external gills are fairly well developed. The opercular fold 

 is beginning to be formed. 



The anterior part of the lumen of the oesophagus is blocked up by 

 epithelial cells. The pulmonary diverticula are small lateral out- 

 growths which extend but a short distance posteriorly from the 

 oesophagus. 



Passing back from the blocked region of the oesophagus the 

 gut makes a curve to the left about the liver and pancreas, and then 



