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. This difference entails important modifications in development, 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 Elasmobranchii and Osseous Fishes a true anus of Rusconi, or primitive opening into the alimentary canal, which is absent in Birds. Yet in neither Elasmobranchii[170] 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[171]. Footnote 170: Vide p. 296 and Plate 9, fig. 1 and 2, and Self, "Comparison," &c., _loc. cit._ Footnote 171: The relation of the anus of Rusconi and blastopore in Elasmobranchii 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. Owing also to the respective situations of the embryo in the blastoderm, the alimentary and neural canals communicate posteriorly in Elasmobranchii and Osseous Fishes, but _not_ in Birds. Of all these points Professor Haeckel makes no mention. The support of his views which Prof. Haeckel attempts to gain from Götte's researches in Mammalia is completely cut away by the recent discoveries of Van Beneden[172] and Hensen[173]. Footnote 172: "Développement Embryonnaire des Mammifères," _Bulletin de l'Acad. r. d. Belgique_, 1875. Footnote 173: _Loc. cit._ It thus appears that Professor Haeckel's views but ill accord with the facts of vertebrate development; but even if they were to do so completely it would not in my opinion be easy to give a rational explanation of them. Professor Haeckel states that no sharp and fast line can be drawn between the types of 'unequal' and 'discoidal' segmentation[174]. In the cases of unequal segmentation he admits, as is certainly the case, that the larger yolk cells (hypoblast) are simply enclosed by a growth of the epiblast around them; which is to be looked on as a modification of the typical gastrula invagination, necessitated by the large size of the yolk cells (vide Professor Haeckel's paper, Taf. II. fig. 30). In these instances there is no commencement of an ingrowth in the _manner supposed for meroblastic ova_. Footnote 174: For an explanation of these terms, vide Prof. Haeckel's original paper or the abstract in _Quart. Journ. of Micr. Science_ for January, 1876.