DEVELOPMENT OF VERTEBRATES. 121 



In the next stage (D II) the formation of the alimentary canal 

 (al] has commenced, but it is to be observed that there is in this 

 case no true involution. 



As an accompaniment to the encroachment upon the seg- 

 mentation cavity, which was a feature of the last stage, the 

 cells to form the walls of the alimentary canal have come to 

 occupy their final position during segmentation and without the 

 intermediation of an involution, and traces only of the invo- 

 lution, are to be found in (i) a split in the lower layer cells 

 which passes along the line separating the small and the large 

 lower layer cells ; and (2) in the epiblast becoming continuous 

 with the hypoblast on the dorsal side of the mouth of this split. 

 It is even possible that at this point a few cells (though cer- 

 tainly only a very small number) of those marked blue in 

 D I become involuted. This point in this, as in all other cases, 

 is the tail end of the embryo. The other features of this stage 

 are as follows : (i) The segmentation cavity has become smaller 

 and less conspicuous than it was. (2) The epiblast cells have 

 begun to grow round the yolk even in a more conspicuous 

 manner than they did in the Frog, and are accompanied by a 

 layer of mesoblast cells which again becomes thickened at its 

 edge. The mesoblast cells in the region of the body are formed 

 in the same way as before, viz. by the separation of a layer to 

 form the epithelium of the alimentary canal, the other cells 

 remaining as mesoblast ; and as in the Frog, or in a more con- 

 spicuous manner, we find that the dorsal surface only of the 

 alimentary cavity has a wall formed of a distinct layer of cells, 

 but on the ventral side the cavity is at first closed in by the 

 large spheres of the yolk only. The formation of the ali- 

 mentary canal by a split and not by an involution is exactly 

 what Bambeke finds in Pelobates. 



The next stage, D III, is about an equivalent age to C III in 

 the Frog. It exhibits the same connection between the neural 

 and the alimentary canals as was found there. 



The alimentary canal is beginning to become closed in 

 below, and this occurs near the two ends earlier than in the 

 middle. The cells to form the ventral wall are derived from 

 the large yolk-cells. The non-formation of the ventral wall of 

 the alimentarv canal so soon in the middle as at the ends is an 



