492 



PATTEN AND HAZEN. 



striations {m^. At the posterior side of the first gill are nerve fibers beneath 

 the place where the first gill leaf is forming («.). x 200. 



Fig. 26. Section No. 5. The genital duct and opercular cartilage are larger 

 than in the preceding drawing. SO^ shows at the posterior side of the operculum. 

 SO^ is greatly enlarged, x 200. 



Fig. 27. Section No. 8. The genital duct {g-d) and SO'^ are separated by a 

 thin membrane, x 200. 



Fig. 28. Section No. 10. The genital duct and SO'^ have united. The oper- 

 cular cartilage remains on the dorsal side of the somite. SO'^ and SO^ show a 

 tendency to extend in a posterior direction, x 200. 



Fig. 29. Section No. 19. The somites extend a long distance laterally and 

 posteriorly as closed cavities, x 200. 



Figs. 30-33. Longitudinal sections through the operculum and first branchial 

 appendage of an embryo in which the third gill leaf had commenced to form. 

 The embryo appears older than that of the preceding series as the appendages 

 have lengthened considerably. 



Fig. 30. Section No. i, near the median line. Shows the median end of the 

 genital duct at the base of the operculum. As the somite has grown laterally and 

 the genital duct toward the median line, they no longer appear in the same longi- 

 tudinal sections. Nerve fibers («.) are found at the posterior side of the base of 

 the operculum and the first gill, x 200. 



Fig. 31. Section No. 3. Shows the genital duct with a small lumen and the 

 median edge of the opercular cartilage, x 200. 



Fig. 32. Section No. 10. Shows the lateral end of the genital duct connected 

 with the base of the opercular cartilage. The genital duct is relatively much 

 smaller than in the preceding series and separate from the somite {SO'^'). The 

 first gill has a well-formed cartilage. X 200. 



Fig. 33. Section much farther from the median line. It shows the long, 

 slender cartilage plates ; that of the first gill is attached to the ectoderm on the 

 anterior side of the appendage. A similar condition would be found in other sec- 

 tions in the operculum. The cartilage cells are placed in rows and show a charac- 

 teristic appearance, and take a lighter stain than the mesoderm. The cartilages 

 are surrounded by a thick membrane. Each somite has been transformed into a 

 large venous sinus, and extends from the base of the cartilage through the yolk 

 to the dorsal side of the embryo. A number of nerve fibers and cells («.) are 

 shown in the gill leaf in g.' x 200. 



Fig. 34. Longitudinal section through the operculum and the first gill of a 

 specimen with five gill leaves on the first gill, showing the opercular cartilage 

 attached to the anterior wall of the operculum and to the venous sinus at the base 

 of the appendage. The cartilage is surrounded by a membrane, the perichondrium 

 (/f.). At the apex of the leg slender processes reach from one ectodermic wall 

 to the other. The outer wall is covered by a thin cuticular membrane. X 200. 



