^60 THE ANCESTRY OF VERTEBRATES 



my theory and, at the same time, into better agreement 

 with the facts, resides the ventral ganglia we find, as 

 mentioned above, in Annelids still another group of segment- 

 al ganglia, the lateral or the parapodial ganglia. Now we 

 have traced back already the spinal gantlia of Vertebrates 

 to the ginglia of the ventral nerve-chain in Annelids. What 

 are we to think of the lateral ganglia of the latter in 

 Vertebrates? If they are found here, I think it is only in 

 the branchial region. 



Dermatogenetic part of the cranial ganglia. — In the year 

 1885 it was found simultaneously, but independently, by 

 Beard in Elasmobranchs, by SPENCER in Amphibians and 

 by FRORIEP in Mammals, that, several of the cranial ganglia 

 do not originate, like the spinal ganglia, exclusively from 

 the neural ridge, but that also the lateral epidermis of the 

 head plays here a considerable pnrt. The ganglia referred 

 to are in fact those of the N. trigemenius, the acastico- facialis, 

 the glossopharyngeus and the vagus. The first rudiments 

 •of these ganglia, produced by the neural crest, grow 

 downwards between the ectoderm and the outer side of the 

 myotom, and in Ichthyopsids each fuse with the ectoderm 

 at two places, one above the other: a lateral one, some- 

 what on the level of the notochord, and an epibranchial 

 one, just above the gill-slits. At the point of fusion with 

 the epidermis, thickenings of the latter, placodes, arise, 

 which look much like sense-organs. The corresponding 

 part of the lateral line, which on the head develops earlier 

 than on the trunk, originates afterwards from the lateral 

 series. At the placodes a proliferation of the epidermis 

 5ets in; epidermal cells, dividing actively, are given off 

 to the inside, forming a gan^ilion. At their first appearance 

 these dermatogenetic placode ganglia in some cases are 

 quite independent from the centrogenetic spinal ganglia, and 

 in the frog, according to BEARD (18886, p. 900), they sepa- 

 rate from the epidermis before the spinal ganglia, growing 

 out downwards, have reached them. FRORIEP (1885, p. 37) 

 and BEARD (1885, p. 221) have suggested independently, 

 that the placodes represent rudiments of segmental sense 

 organs, belonging to the lateral line. Much can be said 

 then for the supposition, that the lateral head-ganglia corres- 

 pond to the lateral ganglia of Annelids, the ganglia of the 

 lateral sense-organs. Soon afterwards the dermatogenetic 

 ganglion fuses intimately with the centrogenetic one, so 



