320 J. S. BUDGETT. 



first pouch is between the hyoid and the first branchial arch. 

 The second pouch is between the first and second branchial 

 arch. The third pouch is between the second and third 

 branchial arch (fig. 14, Srd hr.f.). 



The optic bulbs early begin to bud out from the fore- 

 brain, and now just behind the gill-plate is seen the first rudi- 

 ment of the ])ronephros, a slight but defined elevation tapering 

 posteriorly ; mediad to this are seen four or five mesoblastic 

 somites (fig. 14, mes. som.). 



The auditory vesicles are not easily visible until after the 

 appearance of the external gills. 



Up to this time the embryo has lain almost flat upon the 

 surface of the yolk, preserving in all a spherical form ; now, 

 however, it begins to rise from the surface of the yolk, both 

 anteriorly and posteriorly, but the yolk is still nearly spherical 

 (fig. 15). 



The eye-bulbs increase greatly in size, and are exceedingly 

 large in comparison with what is found in Rana at a corre- 

 sponding stage. The ocular muscles are developed very early, 

 and the eye may be seen to be rotated by them on the fourth 

 day of development. A. very conspicuous feature of this stage is 

 the dilated condition of the double gill-pouch (fig. 15). Viewed 

 from the dorsal surface, the head region has now in outline the 

 form of a trefoil. 



The tail now begins to grow back from the surface of the 

 yolk, the dilation of the branchial folds ceases, and in propor- 

 tion to the latter the head portion increases greatly (fig. 16). 



The first pair of external gills now may be seen budding out 

 from the first branchial arch. Below the cleft post-oral region, 

 formed from the sense-plates, the rudiment of the heart is 

 clearly visible (figs. 21, 34, ht.). 



In a side view, more or less transparent as in life, there are 

 to be seen the heart, first pair of external gills, well-formed 

 eye with conspicuous choroid fissure, auditory vesicles, somites, 

 caudal notochord, and extended cloaca. The yolk-sac still 

 retains its spherical form (fig. 24). 



The changes in external form which now take place mainly 



