No. 3.] PHOSPHORESCENT ORGANS. 683 



the inner surface of the epidermis (PI. XLf Figs. 15 and 16). 

 The orientation of the organ is determined in this very early 

 stage. Organs of the ventral line are vertical to the surface 

 of the skin, while organs of the ventro-lateral surfaces are 

 oblique (PI. XLf Figs. 15-20). In embryos 13 mm. long this 

 papilla, in vertical section of ventral organs, presents the 

 general outline of a finger-tip. It has a diameter of about six 

 cells and is four to five cells deep. Pigment cells now appear 

 in the connective tissue beneath the organ and are found in 

 almost every section. In skins they show as the much-branched 

 type of pigment cell spreading over the inner surface of the 

 papilla. In specimens 14 mm. long there are from three to 

 six such cells around each organ. 



The next stage consists in the gradual separation of this 

 papilla from the epidermis. The papilla becomes constricted 

 where its sides are continuous with the general epidermis, the 

 constriction continuing until complete separation occurs. At 

 the same time a new layer of columnar cells forms in the 

 epidermis and all evidence of the former union is obliterated. 

 The separated mass now has a diameter of about .04 mm. and 

 is found in embryos 18 to 20 mm. in length. 



Soon after separation from the epidermis occurs, in fact 

 before in some instances (PI. XXXIX;' Fig. 12, and PI. XL? 

 Fig. 21), the structure elongates slightly in the line of the 

 axis of the developing organ and a differentiation occurs near 

 the base, enabling one to distinguish in the mass two parts, an 

 outer to become the lens and an inner the gland. No separa- 

 tion occurs between the two portions, yet the cells of each 

 become more and more specialized in the direction of the cells 

 of the adult lens and gland, respectively. By the time the 

 embryo becomes free swimming, a length of about 25 mm., 

 the organs possess the general characters of adult organs. 

 Later growth consists in a great increase in size, due to the 

 multiplication of the cells in the lens and gland, respectively. 



Accompanying the differentiation of the lens and the gland 

 there is a corresponding differentiation of the capsule. The 

 connective-tissue cells of the dermis first form a cup-shaped 

 aggregation around the base of the epidermal portion of the 



