FORMATION OF THE LAYERS. 28 1 



vertebrate ova ought to take into consideration the essential differ- 

 ences 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 1 . 



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 2 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 3 . 



Owing also to the respective situations of the embryo in the 



1 I have suggested in a previous paper ("Comparison," &c., Quart. Jotirnal of 

 Micr, 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 abbreviation of 

 the process by which the Elasmobranch embryos cease to be situated at the edge of 

 the blastoderm (vide p. 296 and PI. 9, fig. i, 2). Assuming this to be the real expla- 

 nation of the position of the embryo in Birds, I feel inclined to repeat a speculation 

 which I made some time ago with reference to the primitive streak in Birds (Quart. 

 Journ. of Micr. Science, 1873, p. 280). In Birds there is, as is well known, a struc- 

 ture called the primitive streak, which has been shewn by the observations of Dursy, 

 corroborated by my observations (loc. cit.), to be situated behind the medullary groove, 

 and to take no part in the formation of the embryo. I further shewed 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 Mammals. The position of the primitive streak immediately behind the embryo 

 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 Rusconi in Elasmobranchs. 1 

 put this speculation forward as a mere suggestion, in the hope of elucidating the 

 peculiar structure of the primitive streak, which not improbably may be found to be 

 the keystone to the nature of the blastoderm of the higher vertebrates. 



3 Vide p. 296 and Plate 9, fig. i and 2, and Self, "Comparison," &c., loc. cit. 



3 The relation of the anus of Rusconi and blastopore in Elasmobranchs was fully 

 explained in the paper above quoted. It was there clearly shewn that neither the 

 one nor the other exactly corresponds with the blastopore of Amphioxus, but that the 

 two together do so. Professor Haeckel states that in the Osseous Fish investigated 

 by him the anus of Rusconi and the blastopore coincide. This is not the case in the 

 Salmon. 



B. 19 



