490 



PATTEN AND HAZEN. 



begins with Fig. 9 near the median line and extends laterally almost to the outer 

 edge of the appendages. Borax carmine and Lyon's blue, 10 /x. 



Fig. 9. Section No. i, near the median line, showing five abdominal somites, 

 SO^'^. SO^ is attached to the primitive streak (/.J.) ; the point of separation is 

 indicated by a slight furrow on the dorsal side. The enlargement at the anterior 

 end of the primitive streak will form the sixth somite, x 200. 



Fig. 10. Section No. 3. SO^ extends in a lateral direction beyond the point 

 of attachment to the primitive streak. SO^-'^ are distinct and free from the 

 mesoderm at the base of the appendages. X 200. 



Fig. II. Section No. 6. The median end of the genital duct {g.d^ may be 

 seen in the operculum. SG^ is small and lies close to the surface of the yolk. 

 X 200. 



Fig. 12. Section No. 7. The genital duct is larger than in the previous section, 

 and is nearer SO^. x 200. 



Fig. 13. Section No. 12. The genital duct and SO'^ are in contact. In the 

 first gill the mesoderm cells are arranged in a row, extending down into the mass 

 of mesoderm from the ventral side of the somite to form the cartilage {c.r) of the 

 first gill. X 200. 



Fig. 14. Section No. 13. The cavity of SO'^ and the genital duct are separated 

 by a thin membrane only. X 200. 



Fig. 15. Section No. 14. SO'^ and g.d. have united. A deep furrow on the 

 anterior side indicates the point of union, x 200. 



Fig. 16. Section No. 17. A line of cells on the ventral margin of the opercu- 

 lar somite shows the median limit of the opercular cartilage, x 200. 



Fig. 17. Section No. 20. The opercular cartilage is free from the mesoderm, 

 and consists of a single row of cells on the ventral side of SO^. x 200. 



Fig. 18. Section No. 22. The end of the cartilage rod is broader than before, 

 and a thin membrane connects it with the adjacent mesoderm, x 200. 



Fig. 19. Section No. 26. The opercular cartilage has disappeared. The ven- 

 tral margin of the somite connects with the posterior bunch of mesoderm cells. 

 The somites show a tendency to bend in a posterior direction. X 200. 



Fig. 20. Section No. 32. x 200. 



