Il8 EARLY STAGES IN THE 



smaller cells and a lower one of larger ; in the centre is a 

 segmentation cavity. The stage at the close of segmentation 

 is represented in c I. From the diagram it is apparent that 

 the lower layer cells occupy a larger bulk than they did in the 

 previous animal (Plate 5, B l), and tend to encroach still more 

 upon the segmentation cavity, otherwise the differences between 

 the two are unimportant. There are, however, two points to be 

 noted. In the first place, although the cells of the upper pole 

 are distinguished in the diagrams from the lower by their 

 colour, it is not possible at this stage to say what will become 

 epiblast and what hypoblast. In the second place the cells 

 of the upper pole or epiblast consist of two layers an outer 

 called the epidermic layer and an inner called the nervous. In 

 the previous cases the epiblast consisted of a single layer of 

 cells. The presence of these two layers is due to a distinction 

 which, arising in most other vertebrates late, in the Frog arises 

 early. In most other vertebrates in the later stages of develop- 

 ment the epiblast consists of an outer layer of passive and an 

 inner of active cells. In the Frog and other Batrachians these 

 two layers become distinguished at the commencement of de- 

 velopment. 



In the next stage (c ll) we find that the involution to form 

 the alimentary canal has commenced (al\ but that it is of a 

 very different character to the involution in the previous case. 

 It consists in the growing inwards of a number of cells from the 

 point x (C l) towards the segmentation cavity. The cells which 

 grow in this way are partly the blue cells and partly the 

 smaller yellow ones. At first this involuted layer of cells is only 

 separated by a slit from the remainder of the lower layer cells ; 

 but by the stage represented in c II this has widened into an 

 elongated cavity (al). In its formation this involution pushes 

 backwards the segmentation cavity, which finally disappears in 

 the stage C III. The point x remains practically stationary, 

 but by the general growth of the epiblast, mesoblast and hypo- 

 blast, becomes further removed from the segmentation cavity 

 in C II than in c I. On the opposite side of the embryo to that 

 at which the involution occurs the epiblast cells as before, grow 

 round the lower layer cells. The commencement of this is 

 already apparent in c I, and in c II the process is nearly com- 



