558 LOCy. [Vol. XI. 



PI. XXVI, Fig. i6. The sections 88 to loo lie in the region 

 of the cephalic plate. The following sections, 105 to 112, lie 

 in the neck and trunk region. These sections are remarkable 

 in bringing to light serial depressions along the walls of the 

 neural folds. They show that the serial cup-like differentia- 

 tions extend back of the cephalic plate. I have not been able 

 to determine the number of serial differentiations of this char- 

 acter in the embryo, but it is clear that there are several pairs 

 behind the cephalic plate upon which I have noted, in surface 

 study, four pairs in addition to the primary optic vesicles. 



My sections are not favorable for a critical study of the 

 histological conditions, but it is clear, from them, that a 

 differentiation starts in the anterior patches similar to that 

 mentioned above, for the true eye vesicles. There is a greater 

 frequency of dividing cells and many of the cells become 

 elongated and pear-shaped. 



Comparisons of the two anterior pairs with the eye vesicles 

 give the following points of resemblance. They are formed 

 in precisely the same way, and present the same general 

 appearance ; viewed from within they are all similar concave 

 cups ; they produce corresponding elevations on the outer 

 surface, and, if observed from the outside, they form a series 

 of rounded knobs serially arranged, the eye being the anterior 

 terminations of the row. The marked differentiation in the 

 eye takes place in later stages. There are, however, the 

 histological features spoken of above that serve to distinguish 

 it in early stages, and the accessory vesicle shows the same 

 characteristics. 



These resemblances, coupled with the existence of serial 

 eyes in some of the Invertebrates, has led to the assumption 

 that in Vertebrates these rudiments represent accessory optic 

 vesicles. The fate of the anterior pair would also favor this 

 view ; they pass into the pineal sense-organ, a structure whose 

 visual character is acknowledged. 



If the view expressed above is true, we have a multiple-eyed 

 condition in the embryos of these animals. This is common 

 enough in Invertebrates, but has not been previously noticed 

 in Vertebrates. 



