DEVELOPMENT OF VERTEBRATES. 127 



In the development of a yolk-sac as a distinct appendage, 

 and its absorption within the body, at a later period, the bird 

 fundamentally resembles the dog fish. 



Although there are some difficulties in deriving the type of 

 development exhibited by the Bird directly from that of the 

 Selachian, it is not very difficult to do so directly from Amphi- 

 oxus. Were the alimentary involution to remain symmetrical 

 as in Amphioxus, and the yolk-containing part of the egg to 

 assume the proportions it does in the Bird, we should obtain a 

 mode of development which would not be very dissimilar to that 

 of the Bird. The epiblast would necessarily overgrow the yolk 

 uniformly on all sides and not in the unsymmetrical fashion of 

 the Selachian egg. A confirmation of this view might perhaps 

 be sought for in the complete difference between the types of 

 circulation of the yolk-sac in Birds and Selachians ; but this is 

 not so important as might at first sight appear, since it is not 

 from the Selachian egg but from some Batrachian that it would 

 be necessary to derive the Reptiles' and Birds' eggs. 



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 develope on the type 

 we now find. 



The careful investigation of the development of some Rep- 

 tiles 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 



