126 EARLY STAGES IN THE 



For the possibility of such a change in the date of formation, the 

 early appearance of the nervous and epidermic layers in the Frog 

 affords a parallel. 



The epiblast in its growth round the yolk is only partially 

 accompanied by mesoblast, which, however, is thickened at its 

 extreme edge as in the Frog. Owing to the epiblast not be- 

 coming continuous with the hypoblast at the tail end of the 

 embryo, the alimentary slit is not open to the exterior. The 

 hypoblast is formed by some of the lower layer cells becoming 

 distinguished as a separate layer ; the remainder of the lower 

 layer cells become the mesoblast. 



The formation of the mesoblast and hypoblast out of the 

 lower layer cells has been accepted for the Bird by most ob- 

 servers, but has been disputed by several, and recently by 

 Kolliker. These have supposed that the mesoblast is derived 

 from the epiblast. I feel convinced that these observers are in 

 the wrong, and that the mesoblast is genuinely derived from the 

 lower layer cells. 



The greater portion of the alimentary cavity consists of the 

 original segmentation cavity (vide diagrams). This feature of 

 the segmentation cavity of Birds sharply distinguishes it from 

 any segmentation cavity of other eggs, and renders it very 

 doubtful whether the similarly named cavities of the Bird and 

 of other vertebrates are homologous. On the floor of the cavity 

 are still to be seen some of the formative cells, but observers 

 have not hitherto found that they take any share in forming the 

 ventral wall of the alimentary canal. 



The features of the next stage are the necessary consequences 

 of those of the last. 



The ventral wall of the alimentary canal is entirely formed 

 by a folding-in of the sheet of hypoblast. 



The more rapid folding-in at the head still indicates the 

 previous more vigorous growth there, otherwise there is very 

 little difference between the forms of the fold at the head and 

 tail. The alimentary canal does not of course, at this or any 

 period, communicate with the neural tube, since the epiblast and 

 hypoblast are never continuous. The other features, such as the 

 growth of the epiblast round the yolk-sac, are merely continua- 

 tions of what took place in the last stage. 



