DEVELOPMENT OF TBA.CHIKF8 VIPEBA. 291 



Plate IV. 



Fig. 9a. Side view of above, as seen floating at the surface of the water. X 26. 



10. Early stage of the invagination process, as seen from beneath : («) the 



hickened part which forms the shield in which the keel is afterwards 

 developed. X 26. 



11. Side view of above. x26. 



12. Embryonic sliield well advanced, and just before the keel begins to 



form as a faint longitudinal streak, x 26. 



13. Later stage, in which the anterior portion of the embryonic axis is 

 defined, and the thickening for the optic lobes is seen, x 26. 



(x) Posterior end of embryo turned upwards to eye of observer. 



14. The blastoderm has here spread more than halfway over the yolk, and 

 with it the segmentation-cavity is carried along. X 26. 



15. Embryo of third day, showing thickening where the auditory sacs will 



be formed. x26. 



16. Embryo later on third day, showing four protovertebr£e. X26. 



17. Embryo shortly before closure of blastopore. X 26. 



18. Side view of part of fig. 17, showing the position of Kupffer's 

 vesicle. X 26. 



Plate V, 



Fig. 19. Embryo of fourth day, showing auditory sacs, heart, and 16 proto- 

 vertebrae. X 26. 



20. Side view of preceding figure. X 26. 



21. Embryo of fifth day. x26. 



22. Embryo of sixth day. X^O. 



23. Embryo of seventh day. x 40. 



24. Embryo of eighth day. x40. 



25. Appearance of the heart on the eighth day. 



26. Appearance of the heart on the tenth day. 



Plate VI. • 



Fig. 27. Newly hatched embryo with pigment-spots accurately marked. X 28. 



28. Ventral view of above, showing position of the pectoral and ventral 

 fms, as seen through transparent yolk-sac. X 19. 



29. Embryo three days after hatching. X 28. 



30. Dorsal view of above, showing change in position of the fins, and the 

 further development of pigment. x55. 



LINN. JOTJElSr. — ZOOLOay, VOL. XVLtl, 21 



