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outgrowth in Elasmobranchs to patches of sensory epithelium located 

 on the cephalic plate. 



In a recent article 1 ) I have described and figured embryos of 

 Squalus acanthias, showing accessory eye-cups on the cephalic plate, 

 and in order to make clear what I have to say on the derivation of 

 the pineal eye it will be necessary to restate briefly the points I have 

 made in that article. 



The optic vesicles in Squalus (and, as I have observed, in Galeus 

 also) are formed at a very early stage while the neural plate is broadly 

 expanded and before the edges of the medullary folds begin to rise 

 in any part of their course. A stage, in fact, when the medullary 

 folds of head as well as body are ventrally curved, not having yet 

 attained the horizontal plane in the process of growing upwards. 



At this early stage there may be seen near the extreme anterior 

 margin of the cephalic plate two circular depressions which are the 

 beginning of the optic vesicles. 



Fig. 1 shows at op the appearance of these circular areas when 

 first formed. The embryo from which the figure was made is a very 

 little less than 3 mm long and shows three mesenchymic somites. 



The circular depressions grow deeper and also press towards the 

 lateral margins of the head-plate. The infolding in which they are 

 located extends itself across the median line, but the cup-like de- 

 pressions grow outwards as well as downwards and form evaginations 

 of the lateral walls. The depressions may now be identified with 

 certainty as the primary optic vesicles. After these vesicles become 

 established, tbe cephalic plate behind them, becomes the seat of two 

 pairs of similarly formed cup-like depressions. The latter, both from 

 this method of formation, and from their conformity in structure with 

 the primary optic vesicles , may be fittingly designated accessory 

 optic vesicles. 



Fig. 2 shows their earliest appearance in an embryo a little more 

 than 3 mm long, and showing from surface study 12 mesenchymic 

 somites. 



Fig. 3 represents an embryo with an open neural groove in which 

 the three pairs of eye-like structures are clearly defined. The embryo is 

 placed in such a position that we have a view into the front part of 

 the neural groove, and see the optic vesicles from their external sur- 



1) The Optic Vesicle of Elasmobranchs and their Serial Relation to 

 other Structures on the Cephalic Plate, Jour. Morph., Vol. IX, No. 1, 

 Oct., 1893. 



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