314 F. L. LANDACRE 



Zwischenstrang as described by His is not correct but that in all 

 cases he finds the ganglion arising from a mass of cells derived 

 from the epiblast at the point of the entering angle between epi- 

 blast and cord and that the Zwischenstrang takes no part in its 

 formation and is present after the ganglion has detached itself 

 from the epiblast and that the ganglion anlage can always be 

 detected before the closing in of the medullary plates. Beard 

 failed, however, as his predecessors had failed, to distinguish be- 

 tween the dorso-Iateral placodes and the early stages of the adult 

 lateral line organs. Under the term branchial sense organs he 

 may have been describing either lateral line organs in the head, or 

 dorso-lateral placodes which may give rise to ganglion cells. Von 

 Kupffer, working on Petromyzon, in 1891 made a sharp distinc- 

 tion on the one hand between dorso-lateral placodes or those plac- 

 odes concerned in the origin of the lateral line and lying at the 

 level of the notochord, and on the other hand the epibranchial 

 placodes which arise just over the gill slits. He also states that 

 the epibranchial ganglia are concerned in the origin of the bran- 

 chial nerves, so that one may infer from his work that there were 

 separate origins for what we now designate as lateralis and vis- 

 ceral sensory nerves. 



Miss Piatt ('95) showed that in Necturus the lateral ectoderm 

 gives rise not only to the lateral line ganglia and nerves which 

 appear in three primitive longitudinal ridges, but also to mesecto- 

 derm, cells proliferated from the lateral ectoderm and from the 

 neural crest and assuming the position and characteristics of meso- 

 derm. Miss Piatt seems first to have made a distinction between 

 the earliest thickening of the lateral epidermis, i.e., the dorso-lateral 

 and epibranchial placodes, and the early stages of the lateral 

 line organs. These seem to have been confused by previous 

 workers. She says (p. 500) that "the large dorso-lateral and epi- 

 branchial ganglia are formed from cells that split off en masse 

 leaving the ectoderm external to them for the time thin. A sensory 

 ridge may appear later in the exact place where the ganglion 

 arose, as happens in the supra-orbital line, or sense organs may 

 form at either side of the ganglionic anlage, as in the vagus region." 

 She calls attention (p. 497) to the fact that the growing point of 



