52 Journal of Comparative Neurology. 



are pyriform, with a sharp prolongation which is directed toward 

 the inferior part of the retina. They resemble in a striking 

 manner the embryonic ganglion cells. Embryos 6 mm. long 

 have in the retina some cylindrical cells extraordinarily devel- 

 oped and vividly colored. They have the characteristics of the 

 sensory cells of the sensory organs. At one end they terminate 

 in a blunt point and at the other end they form a prolongation 

 which can be followed even in the more inferior layers of the 

 retina, where it is lost among other filaments which are believed 

 to be nervous. It appears that in this stage a layer of nervous 

 filaments is already found developed in the retina. As there 

 are not at the same time nervous filaments in the stalk of the 

 organ this would indicate that the filaments are developed in a 

 centripetal manner. The sensitive cells mentioned are apparent- 

 ly the same as those described in the stage 5 mm. long, which 

 here have an irregular form. They have the same peculiarities 

 of form and susceptibility. In young Ammocoetes in which the 

 pineal organ is still placed upon the frontal side of the head and 

 inclined obliquely toward the base, upon the frontal side the 

 elements of the retina are found cut longitudinally if the section 

 has been made in a line parallel with them. In these we can 

 follow the union of the elements of the retina with the nerve. 



In Ammocoetes 30 mm. long the retina is fully developed 

 and four layers are found. 



1. Layer of nervous filaments. They pass directly from 

 the nerve into this layer which is placed immediately above the 

 envelopes of the organ. 



2. Layer of basic cells. These are large cells, very dis- 

 tinct, with large nuclei, and colorless protoplasm. This layer 

 is not easily distinguished from the preceeding and even here 

 nervous filaments are present in part. 



3. Layer of nuclei belonging to little cells. 



4. Superior layer of cylindrical cells. These are of two 

 kinds ; one, the rods (" batonnets " — a term employed by Fran- 

 cotte to designate similar cells in Anguis) are distinguished by 

 distinct contours and by susceptibility to picro-carmine, haema- 

 toxylin, and methyl blue. These are without doubt the sensi- 



