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XLIL — SUGGESTION BESPECTING THE EPIBLASTIC 

 OBIGIN OF THE SEGMENTAL DUCT. By A. 

 C. HADDON, M.A., M.E.I.A., Professor of Zoology in 

 the Boyal College of Science, Dublin. (Plate X.) 



[Bead, February 16, 1887.] 



To Dr. Y. Hensen is due the credit of first discovering the epi- 

 blastic origin of the segmental duct in the rabbit (Lepus cuniculus). 

 He first recorded the fact in 1875 (5) ; but the observation appears 

 to have been universally discredited, and even Balfour makes no 

 mention of it in his " Treatise on Comparative Embryology." 

 In 1884 Dr. Gr. F. Spee (11) found that the same occurred in the 

 guinea-pig {Cariacobaya), and in 1886 Professor "W. Flemming (2) 

 confirmed Hensen's account for the rabbit. 



Towards the end of 1886, Dr. J. "W. van Wijhe (13) announced 

 that the segmental duct arose from the epiblast in the thornback 

 ray {Raja clavata), and lastly, Dr. J. von Perenyi (8) has very 

 recently (January, 1887) extended this mode of origin to the frog 

 {Rana esculenta) and to the lizard {Lacerta viridis). 



The origin of the segmental duct from the epiblast being now 

 known to occur in Elasmobranchs, Anura, Lacertilia, and Eodents 

 we are justified in assuming that this is a general and probably 

 primitive mode of formation. With the above-mentioned excep- 

 tions, all embryologists who have recorded observations on the 

 development of the duct agree in stating that it is at first placed 

 immediately below the epiblast, and that it gradually sinks within 

 the mesoblast, until it comes to lie close to the peritoneal epithe- 

 lium ; they also all agree in deriving the duct from the somatic 

 mesoblast. 



The duct arises in the Rodents as a linear proliferation of the 

 epiblast in the region opposite to the intermediate cell-mass 

 (" Grrenzstrang " of Hensen). Flemming points out that the area 

 is of variable length, not even being symmetrical. The separation 

 of this solid cord of cells from the epiblast takes place from before 

 backwards, and first occurs at a time when the mesoblastic somites 



SCIEN. PKOC. R.D.S. — VOL. V. PT. VI. 2 I 



