172 THE FROG. 



be taken as typical of the series. The vessels proper to the 

 arch are derived from four factors : (i) an efferent lacunar 

 vessel, which appears in the mesoblast opposite the base of the 

 external gill, and may be recognised in tadpoles some little 

 time before hatching; (ii) an efferent diverticulum from the 

 dorsal aorta ; (iii) an afferent diverticulum from the truncus 

 arteriosus ; (iv) an afferent lacunar vessel, which lies opposite 

 the base of the external gill, immediately behind the efferent 

 lacunar vessel. 



These four vessels appear in the order given above ; they 

 are at first quite independent of one another ; and, for some time 

 after their first appearance, the lacunar vessels, afferent and 

 efferent, have no connection with any other vessels. 



The efferent lacunar vessel grows rapidly ; it extends 

 dorsally until it meets with, and opens into, the efferent diver- 

 ticulum from the aorta ; and it extends ventrally towards, but 

 not to meet, the truncus arteriosus. It is widest in the middle 

 part of its course, opposite the external gill ; and here it becomes 

 connected with the afferent lacunar vessel by capillary loops in 

 the substance of the gill. This is the condition reached at the 

 time of hatching. 



Shortly after this, the afferent lacunar vessel and the 

 diverticulum from the truncus arteriosus grow towards each 

 other and unite. The circulation in the gill is now definitely 

 established (Figs. 77 and 78) ; the blood passes from the heart 

 to the truncus arteriosus, RT, and from this along the afferent 

 diverticulum and the afferent lacunar vessel, which now form 

 one continuous afferent branchial vessel, AF, to the gill loops, in 

 which it becomes aerated ; from the gill loops it passes along 

 the efferent lacunar vessel and efferent diverticulum, which 

 form a continuous efferent branchial vessel, EF.I, to the dorsal 

 aorta. 



As the external gill increases in size, and becomes fimbri- 

 ated or lobed at its margin, the original capillary loops become 

 lengthened out, and additional ones are developed; but the 

 afferent and efferent vessels remain connected by capillaries 

 alone, and it is only by passing through the gill capillaries that 

 blood can get from the heart to the aorta. 



At a later period of tadpole life, shortly after the mouth 

 opening is established, the internal gills are developed on the 



