PLATE 2 



EXPLANATION OF FIGURES 



3 Reconstruction of Stage XVIII, age 100 hours; length 7 mm. shading and 

 abbreviations as in figure 1. In the preauditory region a thickening has appeared 

 behind the epibranchial placode (horizontal shading). Since the placode ap- 

 peared earlier than the thickening, it is called in the description of this region 

 the posterior extension of the epibranchial placode, but it seems to be homologous 

 with the extensions behind the gill thickenings of the true gills with the excep- 

 tion that they appear before the epibranchial placodes. The preauditory placode 

 is absent and the lateral line primordium is larger than in figure 2. In the post- 

 auditory region the second ectodermal gill thickening is split at its anterior end 

 into two portions of which the more dorsal represents the posterior extension of 

 the second gill and the more ventral the area of contact of the third gill. The 

 posterior portion of this second thickening is the angular thickening. 



4 Reconstruction of Stage XIX; age 106 hours; length 7.3 mm.; shading and 

 abbreviations as in figure 1. This stage shows the first trace of the primordium 

 of the mandibular lateral line, also the first trace of the postauditory lateral 

 line. There are present three gill thickenings posterior to the ear associated with 

 the first, second and third gills. The posterior extension of the third thickening 

 represents the angular thickening. 



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