236 MR GEORGE BROOK ON THE 



may be that in such types the parablast is mainly concerned in the formation 

 of the connective-tissue elements, and so cannot play so important a part in 

 the formation of the germinal layers. In higher vertebrates {e.g., the chick) 

 the mesoblast in connection with the primitive streak is probably formed 

 independently of the parablast, but it is generally admitted that the hypoblast 

 is in part formed from that layer. So far as the chick is concerned, there 

 appears little doubt that the blood and connective-tissue elements are derived 

 from the parablast, and further investigation may show that in the trout and 

 allied types there is a similar double origin of the mesoblast. One further 

 point remains to be noted. The primitive hypoblast, as I have observed it in 

 the herring, is precisely homologous with that of the Amphioxus, which, be it 

 remembered, is a holoblastic type. In both cases the primitive hypoblast 

 becomes differentiated into two lateral plates of mesoblast separated by the 

 notochord, and what remains constitutes the permanent hypoblast. 



LIST OF LITERATURE REFERRED TO IN THIS PAPER. 



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Arts and Sci, vol. xx. 1884. 



2. Boeck. Om Silden og Sildefiskerierne, &c, Christiania, 1871. 



3. Van Bambeke. Recherches sur l'Embryol. d. poissons osseux, Acad. Roy. d. Belgique, Bruxelles, 



1876. 



4. Balfoub, F. M. Treatise on Comp. Embryology, 2 vols., London, 1880-81. 



5. Brook. Prelim. Account of the Devel. of Trachinus vipera, Jour. Linn. Soc. (Zool.), vol. xviii., 



1884. 



6. Brook. On the Origin of the Hypoblast in Pelagic Teleostean Ova, Quart. Jour. Micr. Sci., Jan. 



1885. 



7. Brook. Experiments with Herring Ova, Report of Fishery Board for Scotland. 1885. 



8. Cunningham. On the Relation of Yolk to Gastrula in Teleosteans, Quart. Jour. Micr. Sci., 1885. 



9. Foster and Balfour. Elements of Embryology, 2nd edition. 



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1880 (see also second notice in Comptes Rendus, xcv. 1882). 



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12. His. Entw. des Hiihnchens, Leipzig, 1868. 



13. His. D. Lehre v. Bindesubstanzkeim (Parablast), Archiv f. Anat. u. Phys. (Anat. Abtheil.), 1882. 



14. Hoffmann. Z. Ontogenie d. Knochenfische, Amsterdam, 1881. 



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Hist., 1883. 



16. Klein. Obs. on Development of Common Trout, Quart. Jour. Micr. Sci., 1876. 



17. Kollmann. D. Randwulst u. d. Ursprung d. Stutzsubstanz, Archiv Anat. u. Phys. (Anat. 



Abthiel.), 1884. 



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19. Kupffer. Z. Entw. d. Herings im Ei, Jahresb. d. Comm. z. Wis. Unters. d. deutschen Meere, 



1874-76, Berlin, 1878. 



20. Kupfker. U. Laichen u. Entw. des Herings, Ibid. 



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Sci. Nat., series iv. vol. xvi., 1861. 



22. Meyer. Beob. ii. Wachsthums d. Herings, Jahresb. d. Comm. $c, 1874-76, Berlin, 1878. 



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1872-73. 



24. Ryder. Embryography of Osseous Fishes, Report U.S. Fish. Comm. 1882, Washington, 1884. 



25. RCckert. Z. Keimblattbildung bei Selachicrn, Miinchen, 1885. 



26. Waldeyer. Archiblast u. Parablast, Archiv Milcr. Anat., xxii., 1883. 



