442 CHARLES RUSSELL BARDEEN 



If a well defined neural tube is differentiated, as a rule the tissue 

 in the walls of the tube either in local regions, as for instance gen- 

 erally in the brain, or throughout its entire length, undergoes 

 retrograde metamorphosis as development proceeds, and masses 

 of protoplasm with more or less definite cell boundaries and con- 

 taining more or less clearly degenerate nuclei and pigment are 

 cast into the central canal. In most of these specimens the neural 

 tube in places, especially in the cerebral region, becomes abnorm- 

 ally dilated and, in places, thin walled. Not infrequently the 

 abnormalities are unilateral rather than bilateral. There may 

 be an absence of development of the hind brain or of a portion of 

 the spinal cord on one side, while it is fairly well differentiated 

 on the other side. Sometimes the central nervous system is 

 abnormally dilated at both extremities at an early period (fig. 16). 

 Peripheral nerves. Peripheral nerves are developed onl}^ in 

 those specimens in which the central nervous system is relatively 

 slightly affected and larval differentiation is relatively advanced. 

 Development of the ganglia of the cerebral and spinal nerves 

 seems to be more or less closely associated with the development 

 of a neural tube and in those instances in which a definite neural 

 tube is not found it is usually difficult or impossible to distin- 

 guish sensory ganglia. 



Eye. In most specimens the eye is more or less profoundly 

 affected. In the more extreme forms there is no development of 

 an optic vesicle on either side and apparently no traces of an eye 

 are to be found. In some specimens the optic vesicles may pro- 

 ject toward but not reach the ectoderm and no lens formation is 

 apparent. In the better developed larva? the optic vesicles 

 reach the ectoderm and a rudimentary lens is formed but in most 

 of these specimens the optic stalk and optic cup become abnorm- 

 ally dilated and structural differentiation becomes quite abnormal. 

 A narrow optic stalk containing nerve fibres is found only infre- 

 quently, but occasionally the eyes are relatively well differen- 

 tiated. 



Nose. In the more extreme forms there is no distinct differen- 

 tiation of nasal organs but in those specimens in which olfactory 

 lobes are developed in the brain well marked olfactory pits, as a 



