[63] Further investigations in confirmation of this widely accepted statement are very desirable.
[64] This does not appear to be the case with the anterior opening in Melopsittacus undulatus, though its relations are not clear from Braun’s description (No. [120]).
[65] This nomenclature may seem a little paradoxical. But on reflection it will appear that so long as the embryo is simply extended on the yolk-sphere, the point where the ventral surface begins has to be decided on purely morphological grounds. That point may fairly be considered to be close to the junction of the medullary plate and primitive streak. To use a mathematical expression the sign will change when we pass from the dorsal to the ventral surface, so that in strict nomenclature we ought in continuing round the egg in the same direction to speak of passing backwards along the medullary, but forwards along the primitive streak. Thus the apparent hind end of the primitive streak is really the front end, and vice versâ. I have avoided using this nomenclature to simplify my description, but it is of the utmost importance that the morphological fact should be grasped. If any reader fails to understand my point, a reference to fig. 52 B will, I trust, make everything quite clear. The heart of Acipenser (ht) is there seen apparently in front of the head. It is of course really ventral, and its apparent position is due to the extension of the embryo on a sphere. The apparent front end of the heart is really the hind end, and vice versâ.
[66] Vide Moldenhauer, “Die Entwicklung des mittleren und des äusseren Ohres.” Morphologisches Jahrbuch, Vol. III. 1877.
[67] For details on the development of the allantois the reader is referred to the works of Kölliker (No. [135]), Gasser (No. [127]), and for a peculiar view on the subject Kupffer (No. [136]). In addition to these works he may refer to Dobrynin “Ueber die erste Anlage der Allantois.” Sitz. der k. Akad. Wien, Bd. 64, 1871. E. Gasser, Beiträge zur Entwicklungsgeschichte d. Allantois, etc.
[68] I propose to call these arteries and the corresponding veins the allantoic arteries and veins, instead of using the confusing term ‘umbilical.’
[69] Further investigations are required as to the character of this layer.
[70] For details on this subject vide A. Courty, “Structure des Appendices Vitellins chez le Poulet.” An. Sci. Nat. Ser. III. Vol. IX. 1848.