No. 3.] PHOSPHORESCENT ORGANS. 68 1 



30° to 40°, while in the lower series of the lateral row it is 

 often 80° or more ; in fact the axis is sometimes quite tangent 

 to the surface of the body at that point. In this class of 

 organs a variation from the typical arrangement of the parts 

 may be noted. In a transverse section of such an organ, say 

 from the pleural row, the deeper part of the cup of the reflector 

 extends inward and upward (PI. XXXIXf Fig. 10) ; that is, 

 toward the dorsal part of the fish. It forms a somewhat deeper 

 pocket than is formed by organs on the ventral surface. In 

 such specimens the pigment is amassed around the upper or 

 dorsal portion of the reflector. In these organs on the side 

 of the body the conical base of the lens is relatively longer and 

 is always in the axial line of the organ. The cells of the gland 

 in such organs are arranged radially around the conical part of 

 the lens. 



The organs on the dorsal surface of the body and head have 

 their axes vertical to the surface of the body. All these 

 organs are small and rudimentary and irregular in their develop- 

 ment, as shown by the fact that the lens is small and irregular 

 in form, that the gland and reflector have more ordinary 

 connective tissue in their structure, and especially by the 

 inconstancy in the presence of the dorsal organs in different 

 individual fishes. 



VI. Development of the Phosphorescent Organs. 



The phosphorescent organs arise quite late in the develop- 

 ment of the embryo. In skins of embryos 8.5 to 8.9 mm. 

 in length, and in a transverse serial section of an embryo not 

 measured but probably of about the same length, I find the 

 first or incipient stages in the development of the rows of 

 phosphorescent organs on the ventral surface of the body. 

 The organs appear first inthe ventral, gastric, branchiostegal, 

 and apparently also in the anal rows at the same time. In the 

 anal row it is not easy to determine their first appearance, owing 

 to the changes accompanying the development of the fin. 



In embryos 8 mm. in length the sensory Anlage of the lateral- 

 line system is complete and the sense organs are sufficiently 



