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III. — The Development of the Skull and Visceral Arches in Lepidosiren and 

 Protopterus. By W. E. Agar, B.A., Junior Demonstrator in Zoology at Glasgow 

 University. Communicated by Professor J. Graham Kerr, M.A. (With 

 Three Plates.) 



(MS. received November 8, 1905. Read December 4, 1905. Issued separately August 15, 1906.) 



The material for this work was that collected and prepared by Professor J. Graham 

 Kerr in the Chaco (Lepidosiren), and by Budgett from the Gambia (Protopterus). 

 The process of development follows nearly the same course in the two genera, and 

 where differences occur they are mainly such as are to be expected from a comparison 

 of the adult skulls. While the series of stages of Lepidosiren left no unfilled gaps, the 

 material was scantier in the case of Protopterus. Except where otherwise stated, the 

 detailed descriptions were worked out in Lepidosiren, but whenever I was unable to 

 verify that the process was substantially the same in the other genus, the fact has been 

 mentioned. The stages are numbered in correspondence with the stages figured by 

 Graham Kerr (Phil. Trans. Roy. Soc., B., vol. cxcii.). The Protopterus stages are 

 numbered the same as the corresponding ones in the other genus. As far as possible, 

 the nomenclature adopted by Gaupp * has been adhered to. 



Throughout this work I have been indebted to Professor Graham Kerr for much 

 advice and criticism. 



The Notochord arises from the dorsal wall of the Archenteron,t at first continuous 

 with the mesoblastic rudiment, at about stage 14. It remains in contact with the 

 hypoblast till about stage 24. In stage 23 + the notochord can be seen running 

 forward beyond the front end of the auditory sac. Its anterior end, however, has not 

 the characteristic histological arrangement of the chordal cells, as vertically placed discs, 

 which obtains in the other parts of the notochord at this stage, and soon disintegrates 

 and becomes mesenchymatous in appearance. Before this takes place the primary 

 sheath (elastica externa) has been secreted, so that after the front part of the chorda 

 has disappeared the sheath is left projecting for some way beyond the definitive front 

 end (fig. 1). In consequence of this, although in stage 23 hthe front end of the 

 notochord was in front of the auditory sac, in stage 29 it is about "3 mm. behind it, 

 though the sheath is traceable for a short distance further forward. There is, of 

 course, a certain amount of tissue inside this part of the sheath, but it is exactly like 

 the mesenchyme surrounding it. The definitive notochord can be seen ending quite 

 sharply behind this tissue (fig. 2). In stage 31 (fig. 3) the notochord can be seen 

 growing forward again into this part of the sheath. The advancing tip is pushing 



* " Die Entwickelung des Kopfskeletes," in 0. Hertwig's Handbuch. 

 t Graham Kerr, Quart. Journ. Micr. Sci., vol. xlv. part i. 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART I. (NO. 3). 7 



