No. 3-] THE VERTEBRATE HEAD. 555 



mark the interior of the optic vesicles. PL XXIX, Fig. 65, is 

 a drawing of one of my sections of Torpedo slightly younger 

 than the one of Ziegler's just referred to. The optic vesicles 

 are indicated at op. 



The external appearance of the optic vesicles in later stages 

 is shown in PI. XXVII, Figs. 34, 35, 36, 37. When the stage 

 represented in Fig. 35 has been reached the lens shows exter- 

 nally ; it is formed in the usual vertebrate way. The choroid 

 fissure shows in Figs. 36 and 37. My studies have not been 

 made to include the later stages of development of the optic 

 vesicle, but are confined to its earliest differentiation. 



Sections of the earliest-formed circular areas, show some- 

 thing in the direction of histological differentiation. Mitoses 

 are more frequent and more of the cells are elongated and 

 pear-shaped than in the other parts of the cephalic plate. The 

 differentiation is by no means as marked as in the Rana 

 palustris figured by Eycleshymer — nevertheless indications 

 of change are not wholly lacking. My sections are too thick 

 (lO/Lt to 15/A) for satisfactory histological study, but one can see 

 in them that the middle of the walls of the optic cups are areas 

 of differentiation. The differentiation does not progress far 

 until later, but the frequency of dividing cells, and their 

 change of form continue, in these early stages, to be marks for 

 distinguishing visual epithelium from that of the surrounding 

 brain-walls. The dividing cells are neuroblasts, and these are 

 known to be points from which the nerve-fibres spring. Their 

 presence in any considerable number would, therefore, indicate 

 a differentiation in the direction of increase of sensibility. 



The eyes are the highest developed of the sense-organs, and 

 in that particular stand at the head of the series. But, do the 

 eyes belong to the same series with the other sense-organs, or 

 do they occupy a position by themselves, can they be homol- 

 ogized with the rest } This is a puzzling question, over which 

 there has been much controversy, and upon which there is 

 still much difference of opinion among anatomists. The stock 

 objection to their being classed with the other sense-organs, is 

 this : — they have apparently been derived from a different 

 basis, and their nerves are developed in a different way. The 



