DEVELOPMENT OF VERTEBRATES. 125 



Between the Selachians and the Aves there is a considerable 

 gulf, which it is more difficult satisfactorily to bridge over than 

 in the previous cases ; owing to this I have not attempted to 

 give any intermediate stage between them. 



The first stage of the Bird (F i) is very similar in many 

 respects to the corresponding stage in the Selachian. The seg- 

 mentation cavity is, however, a less well-defined formation, and 

 it may even be doubted whether a true segmentation cavity, 

 homologous with the segmentation cavity in the previously 

 described eggs, is present. On the floor of the cavity which is 

 formed by the yolk are a few larger cells known* as formative 

 cells which, according to Gotte's observations, are derived from 

 the yolk, in a somewhat similar manner to the cells which 

 were formed around the nuclei in the Selachian egg, and 

 which helped to form the ventral wall of the alimentary 

 canal. Another point to be noticed is that the segmentation 

 cavity occupies a central position, and not one to the side as in 

 the Selachian. 



The yolk is proportionately quite as large as in the Sela- 

 chian's egg, but, as in that case, there can be little or no doubt 

 of its being homologous with the largest of the segmentation 

 spheres of the previous eggs. It does not undergo segmentation. 

 The epiblast is composed of columnar cells, and extends a short 

 way beyond the edge of the lower layer cells. 



In the next stage the more important departures from the 

 previous type of development become visible. 



The epiblast spreads uniformly over the yolk-sac and not on 

 the one side only as in the former eggs. 



This is due to the embryo (indicated in F II by a thickening 

 of the cells) lying in the centre and not at the edge of the blasto- 

 derm. A necessary consequence of this is, that the epiblast does 

 not, as in the previous cases, become continuous with the hypo- 

 blast at the tail end of the embryo. This continuity, being of 

 no functional importance, could easily be dispensed with, and 

 the central position of the embryo may perhaps be explained by 

 supposing the process, by which in the Selachian egg the blasto- 

 pore ceases to correspond in position with the opening of the 

 alimentary slit or anus of Rusconi (vide E'), to occur quite early 

 during segmentation instead of at a late period of development. 



