206 PROFESSOR MARSHALL AND EDWARD J. BLES. 



convenient to postpone consideration of the vessels of the hj^oid 

 arch till a later stage. 



The afferent branchial vessel of each arch, according to Maurer, 

 arises from the ventral end of the corresponding efferent vessel close 

 to the truncus arteriosus, and extends upwards along the posterior 

 border of the efferent vessel until it reaches the base of the external 

 gill • here it ends blindly for a time, but soon forms a loop which 

 extends into the gill, and turning back opens into the efferent vessel 

 at its widest part opposite the root of the gill. 



The most important difference between this account and our own 

 is that while Maurer finds a stage in which there is no afferent 

 vessel in the arch, and the efferent vessel forms a direct communi- 

 cation between the heart and the aorta, we find no such stage at all, 

 the afferent vessel being well developed before the branchial vessels 

 have acquired any connection with the heart ; while the connection, 

 when it is^ formed at a later stage, is effected, as we shall describe 

 further on, by the afferent, and not by the efferent vessel. A 

 second point of difference is that, while Maurer describes the 

 afferent vessel of each arch as arising from the ventral end of the 

 efferent arch, and growing up dorsalwards to the gill, we find that 

 its dorsal end is the first part to appear, opposite the gill, that it 

 early acquires connection with the efferent vessel in the gill, and then 

 grows down ventralwards towards the heart, meeting and fusing at 

 a still later stage with an outgrowth from the truncus arteriosus. 



These differences are perplexing and very unwelcome, and we have 

 accordingly studied these stages repeatedly and with great care, and 

 on a considerable number of specimens, in order to make sure of the 

 correctness of our observations. The investigation is a troublesome 

 one, for it is a most difficult matter to determine with satisfactory 

 precision the mode of origin and the exact boundaries of channels 

 which, like the branchial vessels, appear first as irregular lacunar 

 spaces with no proper walls of their own. It is, therefore, quite 

 possible that our account may require modification in minor details, 

 but of its essential correctness we are convinced. 



On the other hand, Maurer's observations have clearly been 

 conducted with great care, and we see no ground for questioning the 

 accuracy of his descriptions. We must therefore conclude that the 

 early stages of formation of the branchial vessels are different in 



