If this view of the Bird's egg be correct, we are compelled to suppose that the line of ancestors of Birds and Reptiles did not include amongst them the Selachians and the Batrachians, or at any rate Selachians and Batrachians which develop on the type we now find.
The careful investigation of the development of some Reptiles might very probably throw light upon this important point. In the meantime it is better to assume that the type of development of Birds is to be derived from that of the Frog and Selachians.
_Summary._--If the views expressed in this paper are correct, all the modes of development found in the higher vertebrates are to be looked upon as modifications of that of Amphioxus. It is, however, rather an interesting question whether it is possible to suppose that the original type was _not_ that of Amphioxus, but of some other animal, say, for instance, that of the Frog, and that this varied in two directions,--on the one hand towards Amphioxus, in the reverse direction to the course of variation presupposed in the text; and on the other hand in the direction towards the Selachians as before.
The answer to this question must in my opinion be in the negative. It is quite easy to conceive the food material of the Frog's egg completely vanishing, but although this would entail simplifications of development and possibly even make segmentation uniform, there would, as far as I can see, be no cause why the essential features of difference between the Frog's mode of development and that of Amphioxus should change. The asymmetrical and slit-like form of involution on the one side and the growth of the epiblast over the mesoblast on the other side, both characteristics of the present Frog's egg, would still be features in the development of the simplified egg.
In the Mammal's egg we probably have an example of a Reptile's egg simplified by the disappearance of the food material; and when we know more of Mammalian embryology it will be very interesting to trace out the exact manner in which this simplification has affected the development. It is also probable that the eggs of Osseous fish are fundamentally simplified Selachian eggs; in which case we already know that the diminution of food material has affected but very slightly the fundamental features of development.