THE DETELOPMENT OF MOTELLA MUSTELA. 307 



Plate VIII. 



Fig. 1. Optical section of the whole egg on the first day, showing segmenta- 

 tion-mass, with the epidermic layer of the epiblast already differen- 

 tiated, and the periblast around the rim of the disk. 



2. Surface view of egg (as floating) just over one day old, showing cell- 

 formation in the embryonal shield, 



2 a. Optical section of same stage, showing formation of the hypoblast 



and free cells in the yolk and on the floor of the segmentation- 

 cavity. 



3. View of the embryonal shield 6 hours later than fig. 2, showing first 

 formation of the embryo. 



3 a. Optical section of fig. 3, showing the notochord. 



4. Surface view of the embryo 5 hours later than fig. 3. 



6. Surface view of embryo about 12 hours later than fig. 4 (early on third 

 day). 



6 a. Caudal end of embryo in fig. 5, enlarged to show the caudal plate 



(Eyder), and the structure of KuplTer's vesicle. 

 6. View of embryo as floating early on 4th day, and 24 hours later than 

 fig. 5. 



Plate IX. 



Pig. 7. Ventral view of newly hatched embryo. 



7 a. Side view of same embryo. Dotted line shows the extension of the 



embryonal fin some hours after hatching. 



8. Ventral view of young fish 1^- day old, 



8 a. Side view of same stage. 



9. Head of young fish 2 days after hatching, showing the cranial flexure 

 and the ventral position of the mouth, 



9 a. Ventral view of the head in same stage, showing the mouth and the 



arrangement of the bony plates. 



Plate X. 



Fig. 10. Anterior portion of a young fish 2^ days old, showing arrangement of 

 pigment and the further development of the cranial flexure. 

 10 a. Dorsal view of same stage. 



11. Anterior end of a young fish ^\ days old, showing further develop- 

 ment of the jaws. 



12. Anterior end of a young fish h\ days old, showing the change in the 

 relative position of the train brought about by the rapid development 

 of the jaws. 



13. Portion of young fish 6^ days old, showing rudiments of circulation, 



14. Dorsal view of anterior portion of young fish in same stage. 



LINIS. JOUEN.— ZOOLOGY, TOL. XTIII, 22 



