9G BULLETIN OF THE 



EXPLANATION OF PLATES. 



In all cross sections the posterior face of the section is the one drawn, so that right 

 and left in tlie figure correspond to right and left respectively in the animal. 



ABBREVIATIONS. 



au. Auditory vesicle. fs. brn. Gill slit. 



Unless otherwise stated, all figures were drawn from material preserved in Klein- 

 enberg's picro-sulphuric mixture, stained in Czokor's cochineal and magnified 

 about 475 diameters. Figures 1-11 and 16 are of cod embryos; Figures 12-15, 

 of ftindulus. 



PLATE L 



Fig. 1. Cross section cut f mm. from anterior end of embryo, three days old, 



2 mm. long. 

 " 2. Cross section of embryo, same age as that of Figure 1 ; cut about i mm. 



posterior to the region of that section. 

 " 3. Cross section cut -^^ mm. from anterior end of embryo, four days old, 



1^ mm. long. 

 " 4. Cross section taken a little posterior to that of Figure 3, from the same 



embryo. 

 " 5. Cross section cut -^^ mm. from anterior end of embryo, Jive and one half days 



old, 1^ mm. long. 



PLATE II. 



Fig. 6. An oblique cross section taken from the region of the forming heart in the 

 same embryo as that of Figure 5, and 90 ft. in front of it. 



" 7. Cross section cut | mm. from anterior end of embryo yn-e dai/s and twenty- 

 two hours old, 2^^ mm. long. 



" 7°. Portion of section shown in Figure 7, magnified about 710 diameters. 



" 8. Cross section cut | mm. from anterior end of embryo, six and one half days 

 old, 2^ mm. long. 



