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SCIENCE PROGRESS 



wall much folded. According to C. Hill the wall of the vesicle 

 in Salmo contains numerous cells resembling ganglion cells, 

 and is connected with the posterior commissure by nerve- 

 fibres, while in more than one genus Studnicka has demon- 

 strated the existence of the usual modified ependymal cells, 

 with projecting, knob-like extremities. 



Whilst almost invariably without any trace of its paired 

 origin in the adult condition, the pineal organ of the true 

 fishes may exhibit in its development strong evidence of an 

 original bilateral symmetry. This evidence is most satisfactory 

 in the case of certain teleost genera and in the ganoid genus 



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Fig. 3. — A. Head of embryo of a bony fish (Coregonus), showing the pineal out- 

 growths (after C. Hill). B. Head of embryo of tuatara (Sphenodoii), showing 

 the pineal outgrowths, etc. (after Dendy). 



Au., auditory vesicle ; C.H., cerebral hemisphere; Ep. 1, anterior (left) pineal or epiphysial 

 outgrowth ; Ep. 2, posterior (right) pineal or epiphysial outgrowth; H.B., hind brain; M.B., 

 mid-brain; Par., paraphysis ; Pros., prosencephalon ; Thai., thalamencephalon. 



Amia, investigated by C. Hill (1891 and 1894), who showed 

 quite conclusively, in the first instance in the genus Coregonns 

 (fig. 3, a), and afterwards in other forms, that the embryo 

 possesses a right and left pineal vesicle, originating as paired 

 outgrowths from the roof of the thalamencephalon, of which 

 the right one gives rise to the adult pineal organ, while the 

 left is vestigial. As in lampreys, however, the left vesicle 

 {Ep. 1) may be placed somewhat more anteriorly than the 

 right one {Ep. 2), so that it is possible to distinguish them 

 as anterior and posterior pineal outgrowths respectively. 



