No. 3-] THE VERTEBRATE HEAD. 585 



EXPLANATION OF PLATE XXVI. 



The figures are from untouched negatives and are noteworthy in showing the 

 very early condition of the optic vesicle, the accessory vesicles and in some cases 

 the primitive metameric segments. They are all photographs oi Sqiialus acanthias 

 and, with the exception of Fig. 7, are X about 20 diameters. 



Fig. I. Photograph of embryo between Balfour's stages B and C. 



Fig. 2. Somewhat older embryo. The embryonic rim on the left side shows 

 faintly some of the metameric segments (not reproduced by the artist). 



Fig. 3. Slightly older embryo to show the formation of the head-plate and the 

 central wedge-shaped process thereon. This is the stage in which the neural 

 folds are started along the margins of the body. They are ventrally curved. 



Fig. 4. Stage with a rounded head-plate when the optic vesicles first become 

 evident. 



Fig. 5. Another embryo, about the same age as the preceding, in which the 

 optic vesicles and one of the accessory vesicles are shown. 



Fig. 6. Somewhat older embryo showing the infolding for the optic vesicles 

 extending across the median plane. 



Fig. 7. Older embryo somewhat higher magnified. The head-plate is very 

 broad, the trunk narrow. The neural folds of the head lie nearly in the horizontal 

 plane. The metameric segments show well on the left margin of the head-plate. 

 They exist in the earlier stages, but are very difficult to catch with the camera. 



Fig. 8. Specimen in which the depressions for the optic vesicles show very 

 distinctly. 



Figs. 9 and 10. Two embryos older (although smaller) than the preceding. In 

 both two pairs of accessory optic vesicles are to be seen on the cephalic plate 

 back of the primary optic vesicles. 



Figs, ii and 12. Two specimens slightly older than the preceding two, seen 

 from different point of observation. 



Fig. 13. Embryo after the neural folds have begun to grow upwards, seen 

 obliquely from the left side. Gives an external view of the vesicles on one side 

 and an internal view of them on the opposite side of the neural folds. 



Fig. 14. Embryo from which Fig. 29, Plate XXVII, is drawn. Shows meta- 

 meric segments on the exposed ventral surface of the neural folds. 



Fig. 15. Embryo of same age as Fig. 13 and Fig. 31, Plate XXVII. Shows 

 metameric segments on the neural folds in front of the eye vesicles. 



Fig. 16. Embryo of the same age as Fig. 9 just above it. Seen in a position 

 more favorable to bring out the accessory vesicles on the cephalic plate. The 

 optic vesicle on the right side shows as an external protuberance. 



Fig. 17. Somewhat older embryo viewed obliquely from above. Shows the 

 optic vesicle of the right side as an external rounded protuberance, and that of 

 the left side from within as a cup. Behind the optic vesicle on the left side of 

 the cephalic plate are four accessory vesicles. They show their serial relation 

 with the primary optic vesicle. 



Fig. 18. Specimen showing a large development of the central tongue-like 

 process. Embryo slightly older than that in Fig. 7. 



