122 



ERNEST CARROLL FAUST. 



pocket so that the bulbous end with its nucleus comes to lie 

 within the cup (Fig. 3). Although the ectodermal cells of this 

 impocketing are at first visible, they appear to play no further 

 part in the development of the organ. Pigment granules are not 

 present until the nerve ending comes to occupy a position within 

 the pocket. Shortly afterward these granules, at first a golden- 

 brown color, come to be laid down between the nerve ending and 

 the ectodermal cup. As the organ matures the cell with its 



FIG. 3. Section through young germ ball of Cercaria urbanensis Cort, showing 

 formation of pigment granules between optic cup and nerve cell, op, optic cell; 

 ope, optic cell nucleus; opn, optic nerve. X 1,460. 



nucleus enlarges and assumes the appearance of a lens, although 

 it does not take on concentric rings. The pigment granules 

 become more numerous and darker in color so that en masse 

 they look black. Meanwhile the ectodermal pocket becomes 

 less and less conspicuous so that in later embryos it entirely 

 disappears. 



While the writer can agree with Looss (1892: 165) that a 

 nucleus is concerned with the formation of the "lens," this 

 nucleus is derived from the nerve center rather than from indif- 

 ferentiated cells in situ. Moreover, the pigment accumulation 

 is not brought about by a secondary nucleus but is the direct 

 result of the apposition of the end organ and the optic cup. Its 

 shape depends on the shape of the optic cup. The melanoidin 

 precipitation is probably a by-product of the nervous system, as 

 has been shown for Cercaria pellucida. 



The eye-spots are, then, specialized end organs around which 

 a considerable amount of pigment accumulates on account of 



