THE NERVOUS SYSTEM OF AMPHIBIA 287 



Specimen e. 60 -, 60 -, 60 -, 60 -, 60 -, 60 -, 60 -, 60 -, 60 -. 



Specimen f. 60 -, 60 -, 35 +, 25 +, 60 -, 55 +, 60 -, 60 -, 25 +, 30 +. 



Specimen g. 60 -, 60 -, 30 +, 60 -, 60 -, 45 -, 50 +, 60 +, 60 +, 35 +. 



Specimen h. 55 +, 60 -, 60 -, 60 -, 60 +, 60 -, 60 -, 60 -, 53 +, 60 -. 



In brief, specimen e gives no evidence of irritability to light, 

 / and g show noteworthy evidence of it, while the other three 

 show slight evidence of it. It may be said, therefore, that at the 

 time swimming begins the retina is just beginning to be irritable 

 to high illumination. 



•The anatomical explanation for these results will not be fully 

 before us till the central nervous system is better understood, 

 but it is important to note here that it is just at this stage that 

 the optic nerve makes its connection with the brain and that 

 the retina is in a very embryonic condition in its outer layers. 

 In the light of these reactions interest attaches to this order of 

 development in the retina. The ganglion cells, it has been noted, 

 are well differentiated and their fibers decussate in the incipicMit 

 chiasma. These neurones have as high a degree of difTcrcMitia- 

 tion as have other neurones that are known to be in the functional 

 condition. The inference, therefore, is that the high and xariable 

 threshold of the retina at this stage is due to the embryonic con- 

 dition of the more peripheral elements in the reflex circuit. On 

 the other hand, it is interesting to know that with the layer of 

 rods and cones and the bipolar cell layer in such an embryonic 

 condition there could be any optic reflexes stimulated. One 

 almost questions whether the ganglion cells themselves ma>' not 

 possess at this time a certain degree of irritability to light. 

 In the early swimming stage, as described in the anatomical part 

 of the paper, mitosis is till going on in the central zone of the layer 

 of rods and cones and only suggestions of neuroblasts can be 

 detected in the region of the layer of bipolar cells. It seems 

 incredible that speciahzed photic receptors and conduction paths 

 can be already- established in such embryonic structures. Upon 

 the basis of studies which are now in progress on the central paths 

 of the optic reflexes it will be necessary to return to more exhaus- 

 tive cytological study of the retina and to an amplification of 



