180 OUTLINES OF CHORDATE DEVELOPMENT 



in character. There is some variation here among different 

 species of Rana; we shall outline the history of these vessels 

 in R. escuknta, as described by Maurer. Here, in each branchial 



ab ig 



FIG. 64. Sections through the branchial region of tadpoles of R. esculenta, 

 showing the development of the gills and the history of the aortic arches. After 

 Maurer. A. 4 mm. larva showing the continuous first branchial aortic arch. 

 B. 5 mm. larva showing the anastomosis between the afferent and efferent 

 portions of the aortic arch. C. 6 mm. larva with vascular loops in the external 

 gills. D. 13 mm. larva. On the left the opercular cavity is closed and the 

 external gill is beginning to atrophy, while on the right this cavity is still open 

 and the external gill well developed and projecting through the opercular open- 

 ing. E. 17 mm. larva. Vessels of the second branchial arch. External gill 

 represented only by a minute pigmented vestige, ai, First branchial aortic 

 arch; ab, afferent branchial artery; ao, root of lateral dorsal aorta; au, auditory 

 organ; c, conus arteriosus; e, epithelioid body; eb, efferent branchial artery; 

 eg, external gill; i, internal (anterior) carotid artery; ig, internal gills; n, nerve 

 cord; o, operculum; p, pharynx; pc, pericardial cavity; r, gill rakers; s, oral 

 sucker; v, velar plate; x, anastomosis between afferent and efferent branchial 

 arteries. 



arch a lacunar vascular space appears (about 4.5 mm.) which 

 early connects ventrally with the truncus arteriosus, and dor- 





