G8 FORMATION OF THE LAYERS. 



Apart from disputed points of development, it appears 

 to me that a comparative account of the development of the 

 meroblastic vertebrate ova ought to take into consideration the 

 essential differences which exist between the Avian and Piscian 

 blastoderms, in that the embryo is situated in the centre of the 

 blastoderm in the first case and at the edge in the second \ 



This difference entails important modifications in develop- 

 ment, and must necessarily affect the particular points under 

 discussion. As a result of the different positions of the embryo 

 in the two cases, there is present in Elasmobranchs and Osseous 

 Fishes a true anus of Rusconi, or primitive opening into the 

 alimentary canal, which is absent in Birds. Yet in neither 

 Elasmobranchs^ nor Osseous Fishes does the anus of Rusconi 

 correspond in position with the point where the final closing in 

 of the yolk takes place, but in them this point corresponds 

 rather with the blastopore of Birds ^ 



Owing also to the respective situations of the embryo in the 

 blastoderm, the alimentary and neural canals communicate 



1 I have suggested in a previous paper {^^ Comparison,'^'' &Q,., Quart. Journal of 

 3Iicr. Science, July, 1875) that the position occupied by the embryo of Birds at 

 the centre, and not at the periphery, of the blastoderm may be due to an abbre- 

 viation of the process by which the Elasmobranch embrj^os cease to be situated 

 at the edge of the blastoderm {vide p. 81 and PI. viii. fig. 1, 2). Assuming this 

 to be the real exi^lanation of the position of the embryo in Lirds, I feel inclined 

 to repeat a speculation which I made some time ago with reference to the primi- 

 tive streak in Birds {Quart. Journ. of Micr. Science, 1873, p. 280). In Birds 

 there is, as is well known, a structure called the primitive streak, which has 

 been shown by the observations of Dursy, corroborated by my observations {loc. 

 clt.), to be situated behind the medullary groove, and to take no part in the 

 formation of the embryo. I further showed that the peculiar fusion of epiblast 

 and mesoblast, called by His the axis cord, was confined to this structure and did 

 not occur in other parts of the blastoderm. Nearly similar results have been 

 recently arrived at by Hensen with reference to the primitive streak in Mam- 

 mals, The position of the primitive streak immediately behind the embiyo 

 suggests the speculation that it may represent the line along which the edges of 

 the blastoderm coalesced, so as to give to the embryo the central position which 

 it has in the blastoderms of Birds and Mammals, and that the peculiar fusion 

 of epiblast and mesoblast at this point may represent the primitive continuity 

 of epiblast and lower layer cells at the dorsal lip of the anus of Kusconi in 

 Elasmobranchs. I put this speculation forwards as a mere suggestion, in the 

 hope of elucidating the peculiar structiu'e of the j)rimitive streak, which not 

 improbably may be found to be the keystone to the nature of the blastoderm of 

 the higher vertebrates. 



2 Vide p. 81 and Plate viii. fig. 1 and 2, and Self, " Comparison,^'' &c., loc.cit. 



3 Tiie relation of the anus of Rusconi and blastopore in Elasmobranchs was 

 fully explained in the paper above quoted. It was there clearly shown that 

 neither the one nor the other exactly corresponds with the blastopore of Amphi- 

 oxus, but that the two together do so. Professor Haeckcl states that in the 

 Osseous Fish investigated by him the anus of Eusconi and the blastoi)ore coin- 

 cide. This is not the case in the Salmon. 



