PLATE 7 



EXPLANATION OF FIGURES 



23 Stage XIII, age 72 hours. A camera outline taken through the middle of 

 the contact of the e?idodermic pharyngeal pocket of the first true gill with the ec- 

 toderm. At this point the ectoderm is not thickened. X 75. 



24 Stage XIII, age 72 hours A camera outline at the posterior limit of the 

 area of contact of the endodermic pharyngeal pocket of the first true gill with 

 the ectoderm. The ectoderm is thickened at the point of contact. X 75. 



25 Stage XIII, age 72 hours. A camera outline taken behind the area of con- 

 tact of pharyngeal endodermic pocket of the first true gill and ectoderm. The 

 section lies near the anterior end of the cavity of auditory vesicle. The epidermis 

 is only slightly thickened in this section, but the thickening is continuous with 

 that shown at the same level in figure 24. X 75. 



26 Stage XIII, age 72 hours. A camera tracing taken one section anterior 

 to the posterior end of the auditory vesicle. The gill thickening extending 

 through the two preceding series cannot be located at the level of this section. 

 The vesicle lies much closer to the neural canal than to the ectoderm. X 75. 



27 Stage XIV, age 76 hours. A camera outline taken posterior to the area of 

 contact of the pharyngeal pocket to the first true gill with the ectoderm. The 

 gill thickening shown in the four preceding figures reaches further ventrally and is 

 continuous with the angular thickening, but no indication of a separation can be 

 seen in this stage. X 75. 



28 Stage XV, age 82 hours. A camera outline taken just behind the area of 

 contact of the pharyngeal pocket of the first true gill. The separation of the com- 

 mon thickening shown in figure 27 has progressed in this stage. The more ven- 

 tral or angular thickening represents the area of contact of the second true gill 

 pocket. X 75. 



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