No. 3.] 



THE VERTEBRATE HEAD. 



553 



obliquely from the left side, and the optic vesicle of that side is 

 seen from the external surface, where it presents the appearance 

 of a rounded eminence. On the opposite side the optic vesicle 

 is seen from within. Cut 9 shows transverse sections of the 

 head of the same embryo. Figs. 16 and 17 show embryos in 

 which the optic vesicle (pp. v) shows from without, and also 

 from within {op.v). In Figs. 19 and 22 the optic vesicle 

 shows clearly from the outside. Transverse sections of the 





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Cut 9. — Eight transverse sections of the embryo shown in PI. XXVI, Fig. 13. x about 30 

 diameters. The accessory optic vesicles (A. op.) have been formed. 



embryo photographed in Fig. 23, after partial closure of the 

 neural groove, are illustrated in cut 10. 



These optic vesicles are so well developed at an early period 

 in Squalus, that it seemed to me probable that similar early 

 formed vesicles might exist in Torpedo ocellata, and have been 

 overlooked by the Zieglers, who have studied the early stages 

 of that form in 1892 ; accordingly I procured embryos of 

 Torpedo ocellata from the Zoological Station at Naples, and 

 studied the head region with some care. The first thing noted 

 was that the embryos of Torpedo ocellata are not nearly as 

 favorable objects for observations as those of Squalus. They 

 are smaller and the beginnings of sense-organs, branchiae, and 

 other structures about the head are by no means so clear as 



