448 JOHN BEARD, 



from the growth and extension of this, as described by Dohrn and 

 myself, that a portion of the system of branchial or lateral sense 

 organs is derived. As the researches of the last seventeen years have 

 shown, this placode, which is at first quite small, grows and increases 

 in one or more directions, and the neuro-epithelium of the placode 

 gives rise to nerve-fibres, ganglion-cells, and sense organs. 



For many years — 15 at least — the writer has felt convinced, 

 that, in addition to growth to form the ordinary sense organs of the 

 so-called suprabranchial series with their associated sensory nerves, 

 the placode also grows downwards along the gill-arch, to which it be- 

 longs, and, therefore, posterior to its cleft. 



It is this downward growth and extension, which cause it to abut 

 upon the thymus-placode, as shown in Figs. 42, 45, 61, and 62. No 

 evidences have been found, that it ever pushes its way through the 

 latter; rather they tend to demonstrate, that the sensory placode 

 works past the thymus one along its posterior margin, in order to get 

 on to the arch. 



In Raja these facts are most easily made out in connection with 

 the placodes of the hyoid and glossopharyngeal arches. Except in 

 the case of the former arch, this downward extension is in ordinary 

 fishes not concerned in the formation of a series of sense organs, 

 but with the production of a sensory portion of the postbranchial 

 nerve. 



At some early period or other — when has not been determined, 

 but its establishment would be a matter of ease — the elongated 

 placode breaks up into two portions, in Froriep's terminology a 

 dorsal and a ventral one, in von Kupfper's a lateral and a epi- 

 branchial. Each of these, of course, retains its original connection with 

 the ganglion, and each may proliferate cells into this ganglion. The 

 result is to furnish pictures like Froriep's text-figure 5, or like some 

 of VON Kupfper's figures in the same "Verhandlungen", or in his 

 later work (1895). 



To sum up briefly, after the original union of the neural or chief 

 portions of certain of the cranial ganglia of Elasmobranchs with the 

 sensory placode on the level of the notochord ^), by extension of the 



1) Somewhere von Kupffer disputes this as happening at this 

 level; but probably the reason be failed to find it in the lamprey was, 

 that his researches did not go back to an early enough period of the 

 development. In Elasmobranchs the sensory placode is at first strictly 

 on the level of the notochord. 



